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Short Run Versus Long Run

Calculating and drawing the short-run cost curves

  • 1 thing to know
  • Unit A2 1

Overview

Average fixed cost, short-run marginal cost curve take the shape it does and average total cost be falling while marginal cost is rising.

What you need to know

Answer each one before you reveal it.

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Key term

Why is the short-run average total cost curve U-shaped rather than continuously falling?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What do average fixed cost, average variable cost, average total cost and marginal cost each mean?

    Model answer

    Average fixed cost is total fixed cost divided by output, average variable cost is total variable cost divided by output and average total cost is total cost divided by output, which also equals average fixed cost plus average variable cost. Marginal cost is the addition to total cost from producing one more unit, calculated as the change in total cost divided by the change in output. Marginal cost can equally be found from the change in total variable cost, since total fixed cost does not change.

  2. 2

    A firm has fixed costs of 60 pounds a week and total variable costs of 40, 64, 78, 96, 130 and 186 pounds at outputs of one to six units. Calculate average fixed cost at each output.

    Model answer

    Average fixed cost is 60 divided by output. It is 60.00 pounds at one unit, 30.00 at two, 20.00 at three, 15.00 at four, 12.00 at five and 10.00 pounds at six units. Average fixed cost falls continuously as output rises because the same fixed sum is spread over more units.

  3. 3

    A firm's total variable costs are 40, 64, 78, 96, 130 and 186 pounds at outputs of one to six units. Calculate average variable cost at each output and identify its minimum.

    Model answer

    Average variable cost is total variable cost divided by output, giving 40 divided by 1 = 40.00 pounds, 64 divided by 2 = 32.00, 78 divided by 3 = 26.00, 96 divided by 4 = 24.00, 130 divided by 5 = 26.00 and 186 divided by 6 = 31.00 pounds. Average variable cost is at its minimum of 24.00 pounds at an output of four units.

  4. 4

    A firm has fixed costs of 60 pounds a week and total variable costs of 40, 64, 78, 96, 130 and 186 pounds at outputs of one to six units. Calculate average total cost at each output and identify its minimum.

    Model answer

    Total cost is 100, 124, 138, 156, 190 and 246 pounds respectively. Dividing by output gives average total cost of 100.00, 62.00, 46.00, 39.00, 38.00 and 41.00 pounds. Average total cost is at its minimum of 38.00 pounds at an output of five units.

  5. 5

    A firm's total cost is 100, 124, 138, 156, 190 and 246 pounds at outputs of one to six units, with total cost of 60 pounds at zero output. Calculate marginal cost for each unit.

    Model answer

    Marginal cost is the change in total cost from one more unit. The first unit costs 100 - 60 = 40 pounds, the second 124 - 100 = 24, the third 138 - 124 = 14, the fourth 156 - 138 = 18, the fifth 190 - 156 = 34 and the sixth 246 - 190 = 56 pounds. Marginal cost falls to a minimum of 14 pounds on the third unit and rises thereafter.

  6. 6

    Show that average total cost equals average fixed cost plus average variable cost using a firm with fixed costs of 60 pounds and total variable cost of 130 pounds at an output of five units.

    Model answer

    Average fixed cost is 60 divided by 5 = 12.00 pounds and average variable cost is 130 divided by 5 = 26.00 pounds, so the two sum to 38.00 pounds. Total cost is 60 + 130 = 190 pounds, and 190 divided by 5 = 38.00 pounds. The two routes agree because dividing the sum of two totals by output is the same as summing the two averages.

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