Cost Theory

Cost Theory

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Cost theory studies how a firm’s production costs behave as output changes, separating explicit cash outlays (wages, rent, raw material) from implicit opportunity costs (owner’s foregone salary, rent on owned land). Economic cost = Explicit + Implicit cost, so economic profit is always lower than accounting profit.

In the short run at least one factor is fixed, giving TFC > 0, while TVC varies with output Q. The must-know formulas are:

• TC = TFC + TVC
• AC = TC / Q = AFC + AVC
• MC = ΔTC / ΔQ
• Break-even: Q* = TFC / (P − AVC)

High-yield for TNPSC Group 1: MC cuts both AVC and ATC at their minimum points, and AC falls when MC < AC and rises when MC > AC.

The shut-down rule in the short run is P ≥ min AVC; in the long run all costs are variable and the firm exits if P < min LAC.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Short run vs long run

In the short run, capital (or some other input) is fixed, so TFC > 0 and the firm carries a positive AFC = TFC / Q, which continuously falls as Q rises. In the long run, every factor is variable and TFC collapses to zero. This is why the LRAC has no fixed-cost component and behaves as a smooth envelope.

Shape of short-run cost curves

Because of the Law of Variable Proportions, MC, AVC and ATC first fall (increasing returns to the variable factor) and then rise (diminishing returns). The result is a U-shaped AVC and ATC, and an MC curve that intersects AVC and ATC precisely at their minimum points.

Curve	Behaviour	Reason
AFC	Monotonically falling, no minimum	TFC spread over more units
AVC	U-shaped	Variable-factor returns
ATC	U-shaped, lies above AVC	AFC + AVC component
MC	U-shaped, cuts AVC & ATC at minima	ΔTC per extra unit

MC–AC relationship

AC falls while MC < AC, is at its minimum when MC = AC, and rises once MC > AC. The same logic applies to AVC. Hence a common TNPSC trap is the claim “AC = MC + AFC” — the correct identity is AC = AFC + AVC.

Long-run average cost

LRAC is the envelope curve tangent to the minimum points of all possible SRAC curves. It slopes down with internal economies of scale (specialisation, bulk buying, indivisibilities, marketing, finance), is flat under constant returns, and slopes up with diseconomies (managerial coordination failures, input bottlenecks).

Break-even and shut-down

A firm breaks even when TR = TC, i.e., P = AC, earning only normal profit. It shuts down in the short run only when P < min AVC, because fixed costs must be paid regardless of output. In the long run, the exit condition is P < min LAC.

Typical TNPSC MCQ patterns

• Numerical computation of MC, AVC or ATC from a cost table.
• Assertion–reason on MC = AC at AC’s minimum.
• Identifying the correct shut-down price from a given AVC schedule.
• Distinguishing accounting profit (TR − explicit cost) from economic profit (TR − explicit − implicit).

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Sunk cost vs fixed cost

A sunk cost is past and irrecoverable (e.g., a spent advertisement campaign) and must be ignored in current decisions. A fixed cost is still incurred each period and remains relevant for the shut-down choice. Mixing the two is a frequent cause of wrong answers in TNPSC case-study items.

Economies and diseconomies of scope

Producing two related goods jointly can lower total cost when joint cost < sum of standalone costs; this defines economies of scope. Diseconomies of scope arise when shared inputs create congestion or quality dilution. Both are tested through short numerical examples: given separate and joint cost figures, the candidate must judge whether scope is present.

Worked micro-example

Suppose TFC = ₹1,000, and TVC at Q = 10 units is ₹600.

• TC = 1,000 + 600 = ₹1,600
• AC = 1,600 / 10 = ₹160
• AFC = 1,000 / 10 = ₹100
• AVC = 600 / 10 = ₹60
• If price P = ₹200, profit π = (200 − 160) × 10 = ₹400 (supernormal)
• Break-even Q* = 1,000 / (200 − 60) ≈ 7.14 units

Common mistakes to avoid

• Treating accounting profit as economic profit and missing implicit rents on owned land.
• Confusing P = AC (break-even) with the long-run equilibrium condition P = MC = min AC under perfect competition.
• Believing LRAC is the minimum of every SRAC; it is only tangent to selected SRACs.
• Setting the supply rule as P ≥ min AFC instead of the correct P ≥ min AVC.

Practice prompts

1. If MC at Q = 50 is ₹40 and AC at the same output is ₹48, will AC rise or fall if Q increases by one unit? Justify using the MC–AC relationship.
2. A farmer owns the land (implicit rent ₹20,000) and invests own capital (foregone interest ₹15,000). Explicit costs are ₹2,00,000 and revenue is ₹2,60,000. Compute accounting profit and economic profit, and state whether the farmer should continue in the long run.

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