AP Microeconomics / Production, Cost, and the Perfect Competition Model
Lesson
From production to cost curves
Diminishing returns turn extra output into extra cost.
Learning goals
- Relate diminishing marginal product to rising marginal cost.
- Distinguish fixed, variable, average, and marginal cost.
Explanation
In the short run, at least one input is fixed. Adding more variable input at first can raise output quickly, then extra workers or hours add less. That fade is diminishing marginal product.
Fixed costs do not change with output. Variable costs do. Marginal cost is the extra cost of one more unit. Average total cost is total cost divided by output. When marginal cost sits below average cost, it pulls the average down; when it sits above, it pulls the average up.
Key terms
- Marginal cost. The additional cost of producing one more unit of output.
- Diminishing marginal product. The decline in extra output from adding one more variable input while other inputs stay fixed.
Common mistakes
- Treating average cost as the cost of the next unit.
- Forgetting that fixed cost is still there when output is zero in the short run.
Practice
Original Marlow Works items. Check the answer explanation after you try.
A firm in a perfectly competitive market faces a market price of $16. Its marginal cost is MC = 4 + 2q. (a) Find the profit-maximizing output. (b) If ATC at that output is $12, find economic profit. (c) Explain why this outcome is not a long-run equilibrium if entry is free.
Original Marlow Works item — not a College Board question.
Take your time—this is practice, not a test.
Answer explanation
If MC is above ATC, what is happening to ATC as output rises?
Answer. ATC is rising because the extra unit costs more than the current average.
Marginal values pull averages toward themselves.
Related resources
External links with reuse status. Marlow Works is independent and does not copy restricted exam or textbook material.
Official / link only
AP Microeconomics course page
Official six-unit framework for markets, costs, and market failure.
College Board · All rights reserved · accessed 2026-10-01
Open sourceCC BY — attribution required
Principles of Microeconomics: Scarcity and Social Provisioning
Open microeconomics background for scarcity, markets, and elasticity.
Open Textbook Library listing · CC BY · accessed 2026-10-01
Open source