# Relationship between short run long cost curves The STC curve cannot cross At the long run cost minimizing level of. Long-Run Average Costs (LRAC) are the lowest short-run Average Total Costs ( ATC) at any given quantity. The LRAC curve is made of the lower-bound of all. Each time, the scale of operations is changed, a new short-run cost curve will have to be drawn for the firm such as SAC', SAC” and SAC” in the next diagram. At any output other than Q0, the short-run input combination is less efficient—would result in a higher cost—than the combination that would be chosen if all inputs were variable. Suppose, for example, that the firm wants to increase its output from Q0 to Q1. If all inputs were variable, it could produce this new output at TCL. Only at output Q0 are the two curves equal.

## Significance of Short-Run and Long-Run Cost Curves in Economics

Given the total cost curves in Figure 13, short-run average cost will be equal to long-run average cost only at an output of Q0. These are the short- run curves for the plant size designed to produce output QS optimally. Since the short-run total cost curve would be tangent to the long-run total cost curve at this output, the two average cost curves are also tangent at this output.

Since marginal cost is given by the slope of the total cost curve, long-run marginal cost equals short-run marginal cost at the output given by the point of tangency, QS. Thus, SRAC3 is increasing at this point also.

### Significance of Short-Run and Long-Run Cost Curves in Economics

The longer the period to which the curve relates, the less pronounced will be the U-shape of the cost curves. By the long period, we mean the period during which the size and organisation of the firm can be altered to meet the changed conditions.

Why is the Long-run Cost Curves Flatter? The answer can be given in terms of fixed and variable costs. We have said before that no costs are fixed in the long-run, i. In other words, the longer the period, the fewer costs will be fixed and the more costs will be variable.

Economies of Scale- Micro 3.2

That is, in the long period, the total fixed costs can be varied, whereas in the short period, this amount is fixed absolutely. In the short-run, if output is reduced, average cost will rise because the fixed costs will work out at a higher figure.

But, in the long-run, fixed costs can be reduced if the output is continued at the low level. Hence, average fixed cost will be lower in the long than in the short run. The variable costs will not rise as sharply in the long-run as in the short-run, because in the long-run, the size of the firm can be increased to deal more economically with an increased output.

Thus, LAC curves are flatter than the short-run cost curves, because, in the long-run, the average fixed cost will be lower, and variable costs will not rise to sharply as in the short period. We have explained above that the long-run cost-curves are U-Shaped. That is, as output is increased, the cost per unit falls, then it reaches a minimum after which it starts ascending so that it takes the shape of U.

How do we account for this U-shape?

### Short Run and Long Run Average Cost Curve

The reason is that the cost curve falls on account of the various economies of scale. But when the firm has expanded too much, economies are changed into diseconomies and the cost curve starts rising. Empirical studies have further revealed that there is relatively very large flat portion or a large horizontal region in the centre of the long-run average cost curve.

This means that for a considerable range of production, the long-run average cost remains the same and then it moves up at the right and making a sort of dish or saucer.

## Cost curve

The long-run marginal cost curve tends to be flatter than its short-run counterpart due to increased input flexibility as to cost minimization. The long-run marginal cost curve intersects the long-run average cost curve at the minimum point of the latter.

Long-run marginal cost equals short run marginal-cost at the least-long-run-average-cost level of production. Graphing cost curves together[ edit ] Cost curves in perfect competition compared to marginal revenue Cost curves can be combined to provide information about firms.

In this diagram for example, firms are assumed to be in a perfectly competitive market. In a perfectly competitive market the price that firms are faced with would be the price at which the marginal cost curve cuts the average cost curve. Cost curves and production functions[ edit ] Assuming that factor prices are constant, the production function determines all cost functions. In this case, with perfect competition in the output market the long-run market equilibrium will involve all firms operating at the minimum point of their long-run average cost curves i.

If, however, the firm is not a perfect competitor in the input markets, then the above conclusions are modified. For example, if there are increasing returns to scale in some range of output levels, but the firm is so big in one or more input markets that increasing its purchases of an input drives up the input's per-unit cost, then the firm could have diseconomies of scale in that range of output levels. Conversely, if the firm is able to get bulk discounts of an input, then it could have economies of scale in some range of output levels even if it has decreasing returns in production in that output range.

If MC equals average total cost, then average total cost is at its minimum value.