Production, Cost & Perfect Competition

Name: AP Microeconomics Unit 3: Production, Cost & Perfect Competition
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22–25% of the exam9 lessons · 126 min57 terms

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What this unit covers

The topics below follow the published Micro course framework for Unit 3. This unit is worth 22–25% of the exam, so budget your time against that rather than against how long the unit takes to teach.

Production functionCostsProfit maximizationPerfect competition

Lessons in this unit

Formulas in Unit 3

Key cost relationships

TC = FC + VC · ATC = TC/Q = AFC + AVC · MC = ΔTC / ΔQ

Marginal cost pulls the averages: when MC is below ATC, ATC falls; when MC is above ATC, ATC rises; MC crosses ATC (and AVC) at their minimums.

Profit and the decision rules

Profit-max: MR = MC · Economic profit = (P − ATC) × Q · Shut down if P < AVC

Produce where MR = MC, then check profit against ATC and viability against AVC. Zero economic profit = normal profit (breaking even including opportunity cost).

The competitive firm’s conditions

Short run: P = MR = MC · Long-run equilibrium: P = MR = MC = minimum ATC (zero economic profit)

A price taker faces a horizontal demand curve, so MR = P. Free entry/exit competes economic profit away to zero in the long run.

Why MC mirrors MP

MC = wage / MP_labor

The wage is fixed, so when marginal product rises, marginal cost falls, and vice versa. MC is MP turned upside down.

The cost identities

TC = TFC + TVC · ATC = AFC + AVC · AFC = TFC/Q · AVC = TVC/Q · MC = ΔTC/ΔQ = ΔTVC/ΔQ

MC can be computed from either total cost or total variable cost, because the difference between them is constant.

Reading the LRATC curve

falling LRATC = economies of scale · flat = constant returns to scale · rising = diseconomies of scale · lowest point = minimum efficient scale

Minimum efficient scale is the smallest output at which LRATC bottoms out. Below it a firm is at a cost disadvantage regardless of how well it is run.

The two profits

accounting profit = revenue − explicit costs · economic profit = revenue − explicit − implicit costs

Economic profit is always smaller. Whenever implicit costs are positive, a firm can show accounting profit and economic loss simultaneously.

The short-run rules

P ≥ ATC → profit, produce · AVC ≤ P < ATC → loss but PRODUCE (covering variable cost) · P < AVC → SHUT DOWN

The shut-down point is the minimum of AVC. Below it, every unit produced adds to the loss.

The long-run equilibrium condition

P = MR = MC = minimum ATC

Four things equal at once. P = MR comes from price-taking, P = MC from profit maximization, and P = minimum ATC from free entry and exit.

Every term in Unit 3

All 57 terms we publish for Production, Cost & Perfect Competition, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.

Law of diminishing marginal returns: Adding units of a variable input to a fixed input eventually reduces the marginal product of each added unit. A short-run phenomenon, and the reason MC eventually rises.
Profit-maximizing rule: Produce where MR = MC, provided price covers average variable cost. It holds in every market structure — what changes is whether MR equals price.
Short run vs long run in production: In the short run at least one input is fixed. In the long run all inputs vary and firms can enter or exit. The distinction defines which cost curves exist.
Total product: Total output produced by a given quantity of variable input. Rises, then rises more slowly, and can eventually fall.
Marginal product: The extra output from one more unit of variable input: ΔTP / ΔL. Where it peaks, marginal cost is at its minimum.
Average product: Total product divided by units of the variable input. Marginal product crosses average product at its maximum — the same relationship as MC and ATC.
Fixed cost: Cost that does not vary with output — rent, insurance, loan payments. Exists only in the short run, and never affects the profit-maximizing quantity.
Variable cost: Cost that varies with output — labor, materials, energy. Zero when output is zero.
Total cost: Fixed cost plus variable cost. TC = FC + VC.
Average fixed cost: FC / Q. Always falling as output rises, because a fixed sum is spread over more units. It never turns up.
Average variable cost: VC / Q. U-shaped, with its minimum reached before ATC's minimum.
Average total cost: TC / Q, equivalently AFC + AVC. U-shaped, and the gap between ATC and AVC narrows as output rises because AFC is shrinking.
Marginal cost: The change in total cost from one more unit: ΔTC / ΔQ. It equals ΔVC / ΔQ, since fixed cost does not change.
Where MC crosses ATC and AVC: At the minimum of each. When marginal is below average, the average falls; when marginal is above average, the average rises. This is arithmetic, not economics.
Why the cost curves are U-shaped: Increasing marginal returns at low output pull MC down; diminishing marginal returns then push it up. MC is the mirror image of marginal product.
Economies of scale: Long-run average cost falls as the scale of the plant rises, from specialization and spreading fixed costs. Shown on the downward-sloping part of the LRATC curve.
Diseconomies of scale: Long-run average cost rises with scale, typically from coordination and management problems. The upward-sloping part of LRATC.
Constant returns to scale: Long-run average cost is flat over a range of output. Its minimum-cost range defines efficient plant sizes.
Perfect competition: assumptions: Many small buyers and sellers, identical products, free entry and exit, perfect information. Together they make each firm a price taker.
Price taker: A firm with no power to set price, because its output is a trivial share of the market and its product is identical to everyone else's.
Demand curve facing a perfectly competitive firm: Horizontal at the market price, so P = MR = AR = D. This is the single most useful fact in the unit.
Market vs firm graph in perfect competition: The market graph has a downward-sloping demand curve and sets the price. The firm graph takes that price as a horizontal line. Questions almost always require both, side by side.
Why not produce where MR > MC: Each additional unit adds more to revenue than to cost, so profit is still rising. Stopping early leaves money on the table.
Calculating profit on a graph: Profit per unit is P − ATC at the profit-maximizing quantity; total profit is that difference times quantity, drawn as a rectangle. Use ATC, never AVC or MC.
Shutdown rule: Shut down in the short run if price is below minimum AVC, because the firm then loses more than its fixed cost by operating. At P above minimum AVC it operates at a loss and covers part of fixed cost.
Short-run supply curve of a competitive firm: The portion of its marginal cost curve above minimum AVC. Below that point the firm supplies nothing.
Break-even point: Where price equals minimum ATC. The firm earns zero economic profit — normal profit — and has no incentive to enter or exit.
Long-run adjustment with economic profit: Profits attract entry, market supply shifts right, price falls, and firms earn zero economic profit again. The mechanism, not just the result, is what earns credit.
Long-run adjustment with economic loss: Losses cause exit, market supply shifts left, price rises, and surviving firms return to zero economic profit.
Long-run equilibrium in perfect competition: P = MR = MC = minimum ATC. Both productively efficient (lowest cost) and allocatively efficient (P = MC) — the benchmark every other structure is judged against.
Constant-cost industry: Entry does not bid up input prices, so the long-run supply curve is horizontal and the long-run price returns exactly to its original level.
Increasing-cost industry: Entry bids up input prices, so long-run supply slopes upward and the new long-run price is higher than the original.
Sunk cost: A cost already incurred and unrecoverable. It should never influence a forward-looking decision, which is why fixed cost does not enter the shutdown comparison in the way students expect.
Productive vs allocative efficiency in perfect competition: Productive efficiency is producing at minimum ATC; allocative efficiency is producing where P = MC. Perfect competition achieves both in long-run equilibrium; no other structure achieves either.
Marginal revenue: The change in total revenue from selling one more unit. Equal to price only when the firm is a price taker; below price whenever the demand curve slopes down.
Total revenue: Price times quantity. Rising while marginal revenue is positive, maximized where marginal revenue is zero.
Reading a cost table: Compute MC as the change in TC between rows, and ATC as TC divided by that row's Q. Mixing the two — dividing a change, or differencing an average — is the standard error.
Why a firm produces at a loss in the short run: If price exceeds minimum AVC, revenue covers all variable cost plus part of fixed cost. Shutting down would mean losing the whole fixed cost instead.
Diminishing returns needs a fixed input: It is a SHORT-RUN idea: a variable input crowds a fixed one. In the long run everything scales, which is why diseconomies of scale is a different concept.
Where diminishing returns begins: At the worker AFTER the one with the highest marginal product. Total output is still rising throughout.
Why MC mirrors MP: MC = wage ÷ MP with the wage fixed, so MC is minimized exactly where MP is maximized. The U-shape of MC is diminishing returns upside down.
The cost identities: TC = TFC + TVC · ATC = AFC + AVC · MC = ΔTC/ΔQ = ΔTVC/ΔQ. Everything derives from total fixed and total variable cost.
Why AFC falls forever: AFC = TFC/Q with a constant numerator, so it declines without limit. TOTAL fixed cost is constant; AVERAGE fixed cost is not.
Why MC cuts averages at their minimums: A marginal value below the average pulls it down and above it pushes it up, so the crossing must be at the minimum. True of any marginal-average pair.
LRATC as an envelope: The lower boundary of all possible short-run ATC curves — for each output, the cost of the best-suited plant. It can never lie above any of them.
Minimum efficient scale: The smallest output achieving the lowest LRATC. Below it a firm is at a cost disadvantage no matter how well it is run.
Diminishing returns vs diseconomies of scale: Returns: short run, a variable input crowds a fixed one. Scale: long run, everything scaled up and average cost still rises. Read for whether an input is fixed.
Explicit vs implicit costs: Explicit: payments made. Implicit: the value of owner-supplied resources — forgone salary, forgone rent, forgone return on invested savings.
Why a firm can show accounting profit and economic loss: Accountants omit implicit costs. A $50,000 accounting profit against a $65,000 forgone salary is a $15,000 economic loss.
What zero economic profit means: A normal profit — the owner earns exactly what their resources could earn elsewhere. Not failure, and the reason nobody enters or exits.
The shut-down rule: P ≥ ATC: profit. AVC ≤ P < ATC: loss, but PRODUCE — variable cost is covered and fixed cost is sunk. P < AVC: shut down.
Why minimum AVC is the boundary: Below it, each unit brings in less than its variable cost, so producing makes the loss worse than the fixed cost alone.
Shut down vs exit: Shutting down is short-run: produce zero, still own the plant, still pay fixed cost. Exiting is long-run: sell the plant, pay nothing.
The competitive firm supply curve: Its MC curve ABOVE minimum AVC. The truncation is exactly the shut-down rule drawn on the graph.
The long-run equilibrium condition: P = MR = MC = minimum ATC. Price-taking gives the first, profit maximization the second, free entry the third.
Why entry and exit drive profit to zero: Positive profit attracts entry, expanding supply and lowering price. Losses cause exit, contracting supply and raising it. Zero is the resting point.
Why perfect competition is doubly efficient: Minimum ATC is productive efficiency; P = MC is allocative efficiency. It achieves both, which is why it is the benchmark for judging monopoly.

What examiners penalize here

Remember the marginal-average rule: when **MC < ATC, ATC falls**; when **MC > ATC, ATC rises**; and **MC crosses ATC and AVC at their minimum points**. This lets you locate the minimum of the average curves directly on a graph.
Run the two-step firm analysis: (1) find output where **MR = MC**, then (2) compare price to **ATC** for profit/loss and to **AVC** for the shutdown decision. Zero economic profit is *breaking even* (normal profit), not a loss — a distinction the exam tests directly.
Draw the competitive firm with a **horizontal demand = MR = price** line and the U-shaped ATC/AVC/MC curves. In the long run, that price line is **tangent to minimum ATC**. Free-response prompts reward showing entry/exit driving profit to zero.
When a table gives total product, write the marginal product column immediately, then divide the wage by each entry for marginal cost. Both columns are usually needed and computing them once saves recomputation under time pressure.
When a cost table has gaps, fill in TC and TVC first, then derive the averages and marginals. Working out of order leads to using an unfilled cell, which propagates through every later answer.
Read whether the scenario holds an input fixed. "More workers in the same plant" is short-run diminishing returns; "a larger plant" or "all inputs doubled" is long-run scale. The two have different answers and the wording is the only clue.
List explicit and implicit costs in two separate columns before computing either profit. Free responses award the two figures separately, and the usual failure is a correct accounting profit followed by an economic profit missing one implicit item.
Show the loss both ways — producing and shutting down — when a free response asks whether a firm should continue. The comparison is the argument, and stating only the rule without the two loss figures often loses a point.
Free responses often ask for both the short-run and long-run outcome. Answer them as two separate stages, and state explicitly that entry or exit is what moves the market between them — the mechanism is usually its own point.

Practice Micro

Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.

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Questions about this unit

How much of the AP Microeconomics exam is Unit 3?

Unit 3, Production, Cost & Perfect Competition, is worth 22–25% of the Micro multiple-choice section according to the published course framework. Across all 6 units that makes it one of the heaviest units on the exam, and worth front-loading.

What topics are covered in Micro Unit 3?

Production, Cost & Perfect Competition covers Production function, Costs, Profit maximization and Perfect competition. We publish 57 terms with definitions for this unit, all of them on this page.

How should I study Micro Unit 3?

Read the 9 lessons below first — about 125 minutes — then drill the 57 terms in cram mode until you can produce each definition from memory rather than just recognize it. Recognition is what makes a unit feel finished when it is not. Finish with practice questions and read the explanation for every one you get right by elimination as well as the ones you miss.

All 6 units of AP Microeconomics

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