SECO201: Microeconomics 2 Course Notes

SECO201: Microeconomics 2 extends core microeconomic tools into deeper analysis of consumer and producer behavior, market structure, and strategic interaction. While introductory microeconomics often focuses on basic demand–supply reasoning and equilibrium, Microeconomics 2 typically demands more rigorous modelling: constrained choice, intertemporal intuition, cost–output decisions under uncertainty or market power, and equilibrium in games. Across South African universities, the course usually pairs theory with diagrammatic reasoning and applied problem-solving.

These exam notes are designed to help you prepare for typical SECO201-style assessments—midterms, problem sets, and final exams—by systematically covering what lecturers commonly test: mastery of key models, correct interpretation of graphs, and disciplined step-by-step solution methods. Examples are contextualised for South Africa where appropriate (e.g., VAT, labour market frictions, competition policy, and common policy debates), while keeping the underlying theory general.

1. Foundations Refresher: Choice, Constraints, and Utility/Profit Maximisation

Microeconomics 2 frequently assumes you are comfortable with foundational concepts, but it expects you to apply them in more complex settings: different functional forms, binding vs non-binding constraints, multi-stage decisions, and comparative statics. This section rebuilds the “engine room” so later market and strategic sections can be solved accurately.

1.1 Consumer theory: constrained optimisation and demand

A central SECO201 skill is deriving or interpreting demand from a utility maximisation problem. You may be asked to:

• Determine whether a constraint is binding.
• Solve for Marshallian (uncompensated) demand.
• Interpret how demand changes with prices and income using comparative statics.

A common structure is:

1. Maximise consumer utility (U(x_1,x_2))
2. Subject to a budget constraint:
   [
   p_1x_1 + p_2x_2 = m
   ]
3. Choose (x_1, x_2\ge 0)

Lagrangian method

Set up:
[
\mathcal{L} = U(x_1,x_2) + \lambda (m – p_1x_1 – p_2x_2)
]
First-order conditions:
[
\frac{\partial U}{\partial x_1} = \lambda p_1,\quad \frac{\partial U}{\partial x_2} = \lambda p_2
]
Divide them:
[
\frac{\frac{\partial U}{\partial x_1}}{\frac{\partial U}{\partial x_2}} = \frac{p_1}{p_2}
]
This ratio gives a condition for tangency: marginal rate of substitution (MRS) equals price ratio at optimum.

Tangency vs corner solutions

A frequent exam question asks: is the optimal bundle at a corner?

• If utility is strictly increasing and preferences are convex, the optimum usually features tangency (interior solution).
• If constraints or functional form create a corner (e.g., perfect complements like Leontief), demand concentrates on axes.

A practical method for corner decisions:

1. Identify preference type (perfect substitutes, perfect complements, Cobb–Douglas, etc.).
2. Consider the FOCs. If they imply negative quantities or violate non-negativity, check corners: (x_1=0) or (x_2=0).

Example: Perfect complements

Let utility be:
[
U(x_1,x_2) = \min{x_1, x_2}
]
Optimal consumption requires (x_1=x_2). Budget implies:
[
p_1x_1 + p_2x_2 = (p_1+p_2)x_1 = m \Rightarrow x_1=x_2=\frac{m}{p_1+p_2}
]

Example: Cobb–Douglas

Let:
[
U(x_1,x_2)=x_1^{\alpha}x_2^{\beta},\quad \alpha,\beta>0
]
Demand:
[
x_1^=\frac{\alpha}{\alpha+\beta}\frac{m}{p_1},\quad x_2^=\frac{\beta}{\alpha+\beta}\frac{m}{p_2}
]
Interpretation: expenditure shares are constant ((\alpha/(\alpha+\beta)) and (\beta/(\alpha+\beta)))—very testable for comparative statics.

1.2 Comparative statics: income and substitution intuition

Comparative statics means: “What happens to equilibrium consumption/production when an exogenous variable changes?” SECO201 often expects you to connect algebra with economics.

Key comparative statics objects:

• Change in income (m)
• Change in a price (p_1) or (p_2)
• Change in preferences (often less common in exams unless specified)

Normal vs inferior goods

If income increases and demand rises, the good is normal. If demand falls, it’s inferior.

A useful derivative concept:

• Own income elasticity of demand:
  [
  \epsilon_{x,m} = \frac{\partial x}{\partial m}\cdot \frac{m}{x}
  ]
  Positive implies normal; negative implies inferior.

Substitution vs income effects

Some exams use Slutsky decomposition or ask you to interpret substitution effects.

For a good (x) and price (p), the total derivative satisfies:
[
\frac{\partial x}{\partial p} = \underbrace{\frac{\partial h}{\partial p}}{\text{substitution effect}} + \underbrace{\frac{\partial x}{\partial m}\cdot \frac{\partial m^c}{\partial p}}{\text{income effect}}
]
where (h) is Hicksian demand and (m^c) is compensating income.

Even if you do not compute Hicksian demand, the sign patterns matter:

• Substitution effect typically moves demand away from the more expensive good.
• Income effect depends on whether the good is normal or inferior.

1.3 Producer theory: cost minimisation and supply decisions

Microeconomics 2 typically shifts emphasis to firms: not just “how supply works,” but how firms choose inputs and how costs shape output under market structure.

A standard cost-minimisation problem:

• Minimise cost (C=wL+rK)
• Subject to producing (q) with technology (f(L,K)\ge q)

Set up:
[
\min_{L,K} ; wL + rK \quad \text{s.t.}\quad f(L,K)\ge q
]
The optimum satisfies the condition:
[
\frac{MP_L}{MP_K}=\frac{w}{r}
]
This equates marginal product ratio to input price ratio.

1.4 Profit maximisation and marginal analysis

Profit maximisation:
[
\pi(q)=p\cdot q – C(q)
]
FOC:
[
\frac{d\pi}{dq}=p – MC(q)=0 \Rightarrow p=MC(q)
]
So if a firm is price taker, supply is determined by where price equals marginal cost, provided price exceeds minimum average variable cost in short-run.

Short run vs long run

• Short run: at least one input fixed (e.g., capital). Firms have a fixed cost (F).
• Long run: all inputs variable; there are no fixed costs in the same sense. Firms can enter/exit markets in competitive equilibrium.

A common exam logic:

• If (p < AVC), producing yields losses beyond fixed costs—firm may shut down.
• If (p \ge AVC) but (p < ATC), firm produces but earns negative economic profit.
• If (p \ge ATC), economic profit non-negative.

Concrete example with numbers

Suppose:

• (ATC(q)=q^2-4q+10)
• (AVC(q)=q^2-2q+6)

At a given price (p), you evaluate:

1. Find output (q) such that (p=MC(q)) (you need MC from cost function).
2. Check whether that (q) is feasible and whether (p\ge AVC(q)).
3. Decide: produce vs shut down; and interpret economic profit using (p-ATC).

2. Elasticity, Welfare, and Market Failure Logic (Plus Application to Competition Policy)

Many SECO201 exams test not only “solve a model,” but “interpret consequences.” Elasticity is a bridge between theory and welfare implications. Market failure logic (externalities, market power, public goods, information asymmetry) appears as conceptual and sometimes quantitative questions.

2.1 Price elasticity of demand and supply

Elasticity measures responsiveness.

Point elasticity

For demand (x(p)):
[
E_p = \frac{dx}{dp}\cdot \frac{p}{x}
]
A negative value for normal downward-sloping demand is common; the magnitude matters.

Arc elasticity (for discrete changes)

If price changes from (p_0) to (p_1) and quantity from (x_0) to (x_1):
[
E=\frac{\Delta x}{\Delta p}\cdot \frac{\bar{p}}{\bar{x}}
]
where (\bar{p}=\frac{p_0+p_1}{2}) and (\bar{x}=\frac{x_0+x_1}{2}).

Implications for tax incidence and revenue

• If demand is inelastic, consumers bear a larger burden in terms of quantity reduction is smaller; revenue considerations depend on elasticity.
• If demand is elastic, quantity changes substantially with price.

A classic welfare intuition:

• Large deadweight loss occurs when demand and supply are both relatively elastic because quantity changes dramatically.

2.2 Elasticity and MR/competition logic (revenue and marginal thinking)

If a firm faces demand (x(p)), total revenue:
[
TR(p)=p\cdot x(p)
]
Marginal revenue (MR) connects to elasticity in monopolistic contexts:
[
MR = p\left(1+\frac{1}{E}\right)
]
If demand is elastic ((|E|>1)), (MR>0); when demand is unit elastic ((|E|=1)), (MR=0). These are frequent conceptual exam points.

Example interpretation

Suppose demand elasticity at current price is (-2). Then:
[
MR = p\left(1+\frac{1}{-2}\right)=p(1-0.5)=0.5p>0
]
So increasing quantity (via lowering price) increases revenue.

2.3 Consumer and producer surplus (CS/PS) and welfare analysis

Welfare analysis is a major exam theme.

Definitions

• Consumer surplus (CS): area between demand curve and price line.
• Producer surplus (PS): area between price line and supply (or marginal cost) curve.

Deadweight loss arises when a policy or distortion prevents trades that would otherwise occur.

Transfers vs efficiency

In many exams, you must separate:

• Transfer (e.g., tax shifts surplus between CS and PS)
• Efficiency loss (DW loss: trades that should happen do not)

2.4 Taxes: incidence and deadweight loss

Consider an ad valorem or specific tax. Microeconomics 2 usually emphasises specific taxes because incidence can be derived with demand and supply elasticities.

With supply and demand:

• Tax (t) creates wedge between consumer price (p_c) and producer price (p_p):
  [
  p_c = p_p + t
  ]

Key exam insights:

• The party with more inelastic side bears relatively more tax burden (in terms of effective price increase or reduced price received).

Stylised elasticities and incidence

• If demand is very inelastic and supply elastic: consumers bear most of tax.
• If supply is very inelastic and demand elastic: producers bear more.

2.5 Subsidies: reverse logic

Subsidies reduce price paid by consumers or increase price received by producers. Welfare effects:

• Government spending is a transfer plus possible efficiency gain.
• Deadweight loss can arise if subsidy causes overproduction relative to the socially optimal level.

If there is a pre-existing distortion (like a tax), subsidy may partially correct. If there is no distortion, subsidy usually causes DWL from overconsumption/overproduction.

2.6 Externalities: private vs social marginal analysis

Externality is a cornerstone of Microeconomics 2 because it directly uses marginal welfare.

Negative externality (e.g., pollution)

Let social marginal cost exceed private marginal cost:
[
SMC(q) = PMC(q) + MEC(q)
]
where MEC is marginal external cost.

Social optimum:
[
Demand = SMC \Rightarrow MB(q) = SMC(q)
]
Market equilibrium:
[
Demand = PMC \Rightarrow MB(q) = PMC(q)
]
Thus market produces too much (relative to social optimum).

Positive externality (e.g., education)

Then:
[
MSB(q) = MPB(q) + MEB(q)
]
Market underproduces relative to social optimum.

2.7 Policy instruments: Pigouvian tax/subsidy, regulation, and cap-and-trade

Pigouvian tax

Set tax equal to marginal external cost at the efficient quantity:
[
t^* = MEC(q^)
]
Then private firms internalise external cost and produce at (q^).

Command-and-control (standards)

Regulators set quantity restrictions or emissions standards.
Pros: certainty in compliance, simpler.
Cons: inflexibility, might be costly if firms have different abatement costs.

Tradable permits (cap-and-trade)

Set a cap on total emissions. Firms trade permits.
Pros: cost-effective because marginal abatement can differ.
Cons: requires monitoring and credible institutions.

2.8 Market power and welfare: monopoly, deadweight loss, and dynamic considerations

Even if later sections cover market structure in detail, it’s useful to link welfare to market power here.

Monopoly:

• Chooses output where (MR=MC).
• Sets price from demand curve.
• Results in:
  • higher price,
  • lower quantity,
  • deadweight loss (underconsumption).

However, SECO201 may also examine potential counterpoints:

• Monopoly may fund R&D or enjoy economies of scale.
• Some “market power” can arise from innovation (dynamic efficiency), not just static inefficiency.

3. Market Structures and Microeconomic Equilibrium: Perfect Competition to Monopoly to Oligopoly

This section is the core “market structures” toolkit. In SECO201, exams often require you to distinguish equilibrium conditions, interpret diagrams correctly, and apply comparative statics within each market type.

3.1 Perfect competition: equilibrium, entry, and long-run outcomes

In perfect competition:

• Firms are price takers.
• Many firms.
• Homogeneous products.

A firm’s decision:

• Short run: produce where (p=MC) if (p\ge AVC); otherwise shutdown.
• Long run: if there is economic profit, entry drives profit toward zero.

Long-run equilibrium

In the long run, competitive equilibrium implies:
[
p = \min ATC
]
Economic profit (=0).

Numerical style practice

If:

• Demand market: (Q=120-2P)
• Firm cost: (C(q)=q^2+10) (hypothetical; need to convert to (MC), (ATC), etc.)
  You typically:

1. Determine (p) where market clears: price sets total quantity.
2. Determine firm’s profit-max output from (p=MC).
3. Compute number of firms (N=Q/q).
4. Check long-run condition if entry exists.

This method is consistent across many exam problems.

3.2 Monopoly: marginal analysis, pricing, and welfare

Monopoly assumptions:

• Single seller.
• Barriers to entry.
• Downward sloping demand.

Monopolist chooses (q_m) such that:
[
MR(q_m)=MC(q_m)
]
Then sets monopoly price (p_m) from demand curve at (q_m).

Elasticity and monopoly markup

A key derived result:
[
\frac{p_m – MC}{p_m} = -\frac{1}{E}
]
where (E) is price elasticity of demand faced by the monopolist.

Interpretation:

• More elastic demand (larger magnitude of (E)) → smaller markup (more competition-like).
• Less elastic demand → bigger markup.

3.3 Price discrimination (if included): simple segmentation logic

If the course includes discrimination, typical cases:

1. First-degree: monopolist captures all CS (theoretically).
2. Second-degree: nonlinear pricing (bulk discounts).
3. Third-degree: separate markets with different elasticities.

For third-degree:
[
\frac{p_i – MC}{p_i} = -\frac{1}{E_i}
]
with (i) indexing market segments. The firm charges:

• higher markups where demand is less elastic.

South Africa context example (generalised)

Consider a firm selling data or services with different consumer segments (e.g., business vs residential). If business users have less elastic demand due to necessity, monopolist pricing would often be higher relative markup in that segment. Exams may test this intuition without requiring country-specific empirical data.

3.4 Oligopoly: strategic interaction and equilibrium concepts

Oligopoly means firms are interdependent. Microeconomics 2 often introduces equilibrium in games even when not framed as “game theory” formally. Expect to use best responses.

3.4.1 Cournot duopoly: quantity competition

In Cournot:

• Firms choose quantities (q_1, q_2).
• Price determined by total quantity.

Market inverse demand:
[
P = a – b(q_1+q_2)
]
Firm (i) profit:
[
\pi_i = P q_i – C(q_i)
]
Typically assume linear costs (C(q_i)=cq_i).

Steps:

1. Write (\pi_i(q_i, q_j)).
2. Differentiate w.r.t. (q_i), set to zero.
3. Get reaction function (q_i=f(q_j)).
4. Solve simultaneously for equilibrium.

Symmetry case (common exam setup)

If (C(q_i)=cq_i) and symmetry holds:
[
q_1=q_2=\frac{a-c}{3b}
]
Total output (Q=2(a-c)/(3b)). Price:
[
P=a-bQ=a-\frac{2(a-c)}{3}=\frac{a+2c}{3}
]
This provides a clean comparison:

• Cournot outcome is more competitive than monopoly but less than perfect competition.

3.4.2 Bertrand duopoly: price competition

In Bertrand:

• Firms choose prices (p_1,p_2).
• Demand allocates to lower-priced firm (with tie-breaking rules).

If products are homogeneous and firms have constant marginal cost (c), Bertrand equilibrium yields:
[
p=c
]
This is surprising because it reproduces competitive price (the “Bertrand paradox”). Many courses then explain why real-world pricing differs: capacity constraints, differentiated products, or increasing marginal costs.

3.4.3 Collusion and the role of repeated interaction (if treated)

Oligopolists may collude, especially if repeated games allow future punishment.

Even if full formal repeated-game mathematics is not expected, conceptual points are examinable:

• In repeated interaction, firms can sustain higher profits with credible threats.
• Stability depends on discount factor (\delta) and incentive constraints.

A common exam inequality:
[
\delta \ge \frac{\pi_D – \pi_C}{\pi_D – \pi_P}
]
where:

• (\pi_C): collusive payoff
• (\pi_D): deviation payoff
• (\pi_P): punishment payoff
  Exact payoff labels depend on the textbook, but the logic is: “future losses must outweigh current gain from deviation.”

3.5 Market structure comparison: key takeaways to remember

A quick comparison framework you can map onto diagrams:

• Perfect competition: (p=MC=) lowest sustainable ATC in long run; many firms; entry erodes profit.
• Monopoly: (MR=MC); price above marginal cost; output below efficient level.
• Cournot duopoly: intermediate outcome; more competitors → lower price closer to competitive benchmark.
• Bertrand with homogeneous goods: can yield (p=MC) under assumptions; differentiation or constraints restore market power.

Exams often give a diagram and ask you to identify which market it resembles by examining:

• whether price equals marginal cost,
• whether (MR) lies below demand,
• whether firms earn zero or positive economic profits,
• how many firms exist and whether entry is possible.

4. Game Theory Essentials for Microeconomics 2: Strategic Choice, Nash Equilibrium, and Mechanisms

Even when the course is described primarily as microeconomics, SECO201 commonly uses game theory tools because strategic interaction is central to oligopoly and policy design. This section builds the core equilibrium reasoning you’ll need for problems involving best responses, dominant strategies, Nash equilibrium, and sometimes simple bargaining or auctions.

4.1 Normal-form games and strategies

A normal-form game describes:

• Players: (i=1,\dots,n)
• Actions: each player chooses from a set (A_i)
• Payoffs: each action profile maps to payoffs (u_i(a_1,\dots,a_n))

A typical 2×2 matrix game:

• Player 1 chooses rows (A) or (B)
• Player 2 chooses columns (C) or (D)
• Payoffs listed as ((\pi_1,\pi_2))

Exam skills:

• Identify best responses.
• Determine Nash equilibrium: a strategy profile where each player’s strategy is optimal given the other’s.

4.2 Dominant strategies and iterated dominance

If one action yields higher payoff regardless of what the opponent does, it’s dominant.

Procedure:

1. For each player, compare payoffs across their actions for each opponent action.
2. If an action dominates, eliminate dominated strategy(s).
3. If only one profile remains after iterated elimination, that profile can be the equilibrium (if consistent).

Example concept: Prisoner’s Dilemma logic

Payoff pattern:

• Each player has incentive to defect because defection gives higher payoff whether the other cooperates or defects.
• Unique Nash equilibrium at (Defect, Defect).
• Socially preferred outcome would be (Cooperate, Cooperate), but incentives prevent it.

SECO201 exams sometimes ask:

• “Is cooperation stable in one-shot game?” (Answer: no, due to Nash equilibrium selection.)
• “How can cooperation arise?” (Repeated game, reputation, side payments, regulation.)

4.3 Nash equilibrium: best-response method

For each player:

• Determine best response set to opponent’s action.
• Intersection of best responses gives Nash equilibrium.

In 2×2, you can often read equilibrium from the matrix by locating:

• For Player 1: mark entries that are highest within each column (given Player 2 action).
• For Player 2: mark entries that are highest within each row (given Player 1 action).
• The cells marked by both players (mutual best responses) indicate Nash equilibrium.

4.4 Mixed strategy equilibria

When no pure-strategy Nash equilibrium exists, a mixed strategy equilibrium may arise.

For a 2×2 game with Player 2 mixing between (C) and (D), Player 1 is indifferent between its actions at equilibrium.

Method:

1. Let Player 2 choose (C) with probability (p) and (D) with probability (1-p).
2. Compute Player 1 expected payoff from choosing Row 1 and Row 2.
3. Set them equal to solve for (p).
4. Similarly compute the probability for Player 2 mixing if needed.

Mixed strategies model uncertainty and randomisation; exams may ask for equilibrium probabilities explicitly.

4.5 Oligopoly as games: Cournot and Bertrand in strategic form

Cournot can be expressed as a game:

• Players choose quantities.
• Payoffs are profits.

Bertrand can be expressed as a game:

• Players choose prices.
• Payoffs depend on which price is lower.

Even if your course uses microeconomic derivations rather than game formalism, you should translate between the two frames:

• Reaction functions correspond to best responses.
• Nash equilibrium corresponds to simultaneous best responses.

4.6 Bargaining and incentives (if included)

If SECO201 includes bargaining, common models are:

• Nash bargaining: maximises product of gains over disagreement.
• Alternating-offers: may yield different outcomes depending on discounting.

Typical exam tasks:

• Explain how outside option affects bargaining outcome.
• Derive or interpret the role of disagreement payoff.

Even without full calculations, you should remember that bargaining outcomes depend on:

• the threat points,
• the bargaining power (captured via disagreement payoffs or discount factors),
• the structure of negotiation.

4.7 Mechanism intuition: why markets sometimes fail to reveal preferences

If the course includes mechanisms:

• Buyers/sellers may misreport information.
• Strategic behaviour leads to inefficient outcomes unless mechanisms are designed carefully (incentive compatibility).

While detailed auction theory might be beyond SECO201, basic ideas such as “incentives matter” are often tested through short conceptual questions.

5. Applied Microeconomics Skills for SECO201 Exams: Elasticities, Costs, Market Power, and Welfare Calculations (With South African Study-Relevant Practice)

This section is an integrated practice guide: it teaches how to approach full exam questions end-to-end. Microeconomics exams reward method. You should learn a repeatable workflow for each major question type: welfare with taxes/externalities, cost/profit maximisation, and equilibrium in oligopoly/strategic settings.

5.1 Exam workflow: reading a question and identifying the correct model

A reliable approach for SECO201 problems:

1. Identify the market environment
   • Perfect competition, monopoly, Cournot, Bertrand, externalities, or taxation policy.
2. Identify the objective
   • Firm profit maximisation, consumer utility maximisation, or welfare maximisation.
3. Identify constraints
   • Budget constraint for consumers.
   • Production technology constraint for firms.
   • Policy instrument wedge or regulation constraint for welfare questions.
4. Derive equilibrium conditions
   • (p=MC), (MR=MC), (MB=SMC), etc., depending on the model.
5. Compute quantities and prices
   • Solve system of equations if needed.
6. Check feasibility and interpretation
   • Non-negativity.
   • Shut-down conditions ((p\ge AVC)).
   • Economic profit vs accounting profit.
7. Present welfare results clearly
   • CS, PS, government revenue, DWL.

This approach prevents common mistakes like mixing monopoly (MR=MC) logic with competitive (p=MC), or using private marginal cost when social marginal cost is required.

5.2 Welfare with taxes: step-by-step calculation template

A typical quantitative tax question includes:

• A demand curve: (Q_d(P))
• A supply curve: (Q_s(P))
• A per-unit tax (t)

Steps:

1. Convert supply and demand into inverse forms if needed:
   • Demand: (P = a – bQ)
   • Supply: (P = c + dQ)
2. Find pre-tax equilibrium:
   [
   a – bQ_0 = c + dQ_0 \Rightarrow Q_0
   ]
3. Find post-tax equilibrium:
   • Consumer price (P_c = P_p + t)
   • Inverse demand relates (P_c) to (Q_t)
   • Supply relates (P_p) to (Q_t)
4. Solve for (Q_t), then compute (P_c) and (P_p):
   [
   P_c = P_p + t
   ]
5. Compute:
   • CS loss and PS loss (areas under curves)
   • Government revenue: (t \cdot Q_t)
   • DWL:
     [
     DWL = \text{(lost trades)} \times \text{(wedge-related value)}
     ]
6. Interpret elasticity-based incidence:
   • Use elasticities if explicitly asked, otherwise infer from curve slopes.

Worked stylised example (symbolic but exam-friendly)

Suppose:
[
P = 100 – Q \quad \text{(demand)}
]
[
P = 20 + Q \quad \text{(supply)}
]
Pre-tax equilibrium:
[
100 – Q_0 = 20 + Q_0 \Rightarrow 80 = 2Q_0 \Rightarrow Q_0 = 40
]
Price:
[
P_0 = 100 – 40 = 60
]

Tax (t=10). Then:

• Consumer side: (P_c = 100 – Q_t)
• Producer side: (P_p = 20 + Q_t)
  Wedge: (P_c = P_p + 10):
  [
  100 – Q_t = (20 + Q_t) + 10 \Rightarrow 100 – Q_t = 30 + Q_t
  ]
  [
  70 = 2Q_t \Rightarrow Q_t = 35
  ]
  Then:
  [
  P_c = 100 – 35 = 65,\quad P_p = 20 + 35 = 55
  ]
  Check: (65-55=10) correct.

Welfare:

• DWL = difference in trades times wedge/price-change measure. In simple linear models, DWL typically equals:
  [
  DWL = \frac{1}{2}\cdot t\cdot (Q_0-Q_t)
  ]
  So:
  [
  DWL=\frac{1}{2}\cdot 10 \cdot (40-35)=5\cdot 5=25
  ]
  This formula is extremely testable for linear supply and demand.

5.3 Externalities: turning marginal analysis into efficient outcomes

For externality questions, the exam often asks you to compute:

• market quantity (q_M),
• efficient quantity (q^*),
• welfare loss,
• policy instrument magnitude.

If externalities are included via marginal conditions:

• Negative externality:
  [
  MB(q) = PMC(q) \quad \text{(market)}
  ]
  [
  MB(q) = SMC(q) \quad \text{(efficient)}
  ]
• Positive externality:
  [
  MB(q) = MPC(q) \quad \text{(market)}
  ]
  [
  MB(q) = MSB(q) \quad \text{(efficient)}
  ]

Example with plausible linear functions

Let demand (marginal benefit) be:
[
MB(q)=50-q
]
Let private marginal cost:
[
PMC(q)=10+q
]
Then market equilibrium:
[
50-q = 10+q \Rightarrow 40=2q \Rightarrow q_M=20
]
Suppose marginal external cost is:
[
MEC(q)=0.5q
]
Then:
[
SMC(q)=PMC(q)+MEC(q)=10+q+0.5q=10+1.5q
]
Efficient equilibrium:
[
50-q=10+1.5q \Rightarrow 40=2.5q \Rightarrow q^=16
]
So market overproduces: (q_M=20) vs (q^=16).

A Pigouvian tax per unit at the efficient quantity is:
[
t^=MEC(q^)=0.5(16)=8
]
Many exam answers include a short interpretation: tax equals marginal external cost so that private decisions align with social costs.

5.4 Cost curves and shutdown/profit logic under uncertainty

Some SECO201 tasks incorporate variations:

• fixed costs,
• variable costs,
• and sometimes demand uncertainty.

Even if uncertainty is not fully formalised, you may see questions like:

• how output changes when fixed costs increase,
• whether shutdown occurs under new price.

Key identities:

• (ATC = \frac{TC}{q} = \frac{F}{q} + AVC)
• (MC = \frac{dTC}{dq})

If fixed cost changes, it:

• does not affect (MC),
• does not affect the output chosen by (p=MC) (assuming same (p) and cost function for variable component),
• but does affect (ATC) and thus economic profit and entry/exit.

Shutdown depends on (AVC), not (ATC).

5.5 Monopoly and oligopoly computations: avoiding common traps

Trap 1: Using price elasticity formula incorrectly

If asked for monopoly markup:
[
\frac{p-MC}{p}=-\frac{1}{E}
]
Ensure you use elasticity as negative value or magnitude consistently. In many exams, elasticity is given as magnitude (|E|). Then:
[
\frac{p-MC}{p}=\frac{1}{|E|}
]
State clearly how you treat signs.

Trap 2: Cournot vs monopoly MR confusion

• Monopoly: uses monopoly (MR) based on demand curvature.
• Cournot: each firm faces a “residual demand” because the other firm’s quantity affects market price. The (MR) in Cournot depends on the strategic quantity interaction.

Exam phrasing matters: If the question explicitly says Cournot, you must write profit with total quantity (q_1+q_2) in price.

5.6 Integrated practice example: externality + market power + policy reasoning

A more advanced exam question may combine:

• a firm with market power (monopoly) producing a good that generates a negative externality,
• and a policy instrument that corrects welfare.

A typical structure:

• Monopoly chooses (q) to maximise private profit (p(q)\cdot q – C(q)).
• Social planner chooses based on (SMC).
• Policy chooses tax (t) so that private choice yields efficient output.

Even if you do not compute full curves, you can demonstrate understanding via:

1. Identify market equilibrium for monopoly: (MR=MC).
2. Identify efficient equilibrium: (MB=SMC).
3. Provide Pigouvian tax: (t=MEC(q^*)) (for per-unit tax), or equivalent instrument magnitude.

This type of question is designed to test whether you can switch from private to social margins.

5.7 South African study focus: how course emphasis often appears in exams

South African universities and colleges teaching Microeconomics 2 often align with national or institutional curricula that emphasise:

• mathematical fluency with marginal conditions,
• interpretation of welfare changes (often linked to policy),
• and application of market structure theory to real industries.

Common South African “application contexts” you might see in exam questions or tutorials (without needing local data):

• Energy and utilities where natural monopoly arguments appear.
• Telecommunications where pricing, competition, and regulation are prominent.
• Transport and logistics with capacity constraints (relevant to Bertrand-with-capacity logic).
• Labour markets where externalities and information issues may be mentioned.
• Taxation discussions where incidence and welfare are standard frameworks.

You should not assume exam questions require memorising South African statistics. Instead, you should be ready to plug policy-style numbers into generic models. When VAT or fuel taxes are mentioned, they are usually there as motivation—not necessarily as parameters you must know. Your job is to compute the effects given the curves and tax/transfer amounts in the question.

5.8 Presentation standards that earn marks

SECO201 exam marking often rewards clarity. For each quantitative question, aim to:

• Write down the correct equilibrium condition before solving.
• Show substitution and algebra steps clearly.
• Label which price/quantity you compute (consumer vs producer price, market vs efficient quantity).
• Provide brief economic interpretation (1–2 sentences) after numerical results.

Common interpretation examples:

• “Because (SMC>PMC), the market produces too much relative to the social optimum.”
• “Economic profit is zero in long-run perfect competition due to entry.”
• “Monopoly charges a price above marginal cost because it equates (MR) with (MC), and demand is downward sloping.”

5.9 Summary checklist for SECO201 revision

Use this checklist to self-test before your exam:

Consumer and producer foundations

• Can set up and solve utility maximisation with a budget constraint (including corner solutions)?
• Can interpret Cobb–Douglas vs perfect complements vs perfect substitutes demand?
• Can derive cost minimisation conditions and interpret input price ratios?
• Can apply shutdown rule: produce iff (p\ge AVC) in short run?

Elasticity and welfare

• Can compute point or arc elasticity and interpret magnitude?
• Can calculate CS/PS changes and identify transfers vs deadweight loss?
• Can determine tax incidence using relative elasticity logic?
• Can compute Pigouvian tax/subsidy magnitude from marginal external cost/benefit?

Market structures

• Can solve perfect competition short-run and long-run outcomes?
• Can solve monopoly using (MR=MC) and demand to get price?
• Can interpret monopoly markup using elasticity?
• Can solve Cournot duopoly with reaction functions and identify equilibrium?
• Can explain Bertrand paradox and why differentiation/capacity changes outcomes?

Strategic equilibrium and game theory

• Can identify best responses and Nash equilibrium in 2×2 games?
• Can recognise dominant strategies and apply iterated dominance?
• Can solve for mixed-strategy probabilities via indifference conditions?
• Can translate oligopoly problems into strategic form logic?

Exam method

• Can follow a consistent problem workflow: identify model → write equilibrium condition → solve → interpret.
• Can label variables clearly and avoid mixing consumer/producer price.

Closing perspective

SECO201: Microeconomics 2 is ultimately about disciplined reasoning at the margin—marginal benefit vs marginal cost, private vs social margins, and best responses in strategic environments. If you can consistently apply the correct equilibrium conditions and translate results into clear economic interpretation, you’ll be well prepared for the variety of question styles used across South African universities, colleges, and TVET-related teaching contexts. The most effective preparation strategy is repeated practice with full solutions: not only deriving answers, but also showing the reasoning markers examiners expect—equilibrium conditions, welfare components, and sign/intuition checks that confirm the solution is economically meaningful.