Editing Marginal cost (section)

== Relationship to fixed costs ==
Marginal costs are not affected by the level of fixed cost. Marginal costs can be expressed as ∆C/∆Q. Since fixed costs do not vary with (depend on) changes in quantity, MC is ∆VC/∆Q. Thus if fixed cost were to double, the marginal cost MC would not be affected, and consequently, the profit-maximizing quantity and price would not change. This can be illustrated by graphing the short run total cost curve and the short-run variable cost curve. The shapes of the curves are identical. Each curve initially increases at a decreasing rate, reaches an inflection point, then increases at an increasing rate. The only difference between the curves is that the SRVC curve begins from the origin while the SRTC curve originates on the positive part of the vertical axis. The distance of the beginning point of the SRTC above the origin represents the fixed cost – the vertical distance between the curves. This distance remains constant as the quantity produced, Q, increases. MC is the slope of the SRVC curve. A change in fixed cost would be reflected by a change in the vertical distance between the SRTC and SRVC curve. Any such change would have no effect on the shape of the SRVC curve and therefore its slope MC at any point. The changing law of marginal cost is similar to the changing law of average cost. They are both decrease at first with the increase of output, then start to increase after reaching a certain scale. While the output when marginal cost reaches its minimum is smaller than the average total cost and average variable cost. When the average total cost and the average variable cost reach their lowest point, the marginal cost is equal to the average cost.