The inflexion point is a point where the sign of curvature changes on the curve. The marginal average cost is the derivative of the cost function. The marginal average cost shows the change in average cost when the per-unit cost is affected due to producing one more unit of a good. The inflexion point is found everywhere on the curve when change occurs on the curve in the context of shape.

The average cost is U shape due to the law of variable proportion. The changes in the shape occur due to a change in the marginal average cost. In the first phase, marginal average cost declines due to the efficient utilization of resources. In the second phase, the marginal average cost becomes constant. From the second phase, curvature changes that lead to a change in the shape of the curve. In the third phase, marginal average cost increases due to inefficient optimum combination that leads to a rise in average cost leading to change in the shape of the curve.

In the output curve, the total product is defined by marginal product in which TP first rises at an increasing rate when marginal product rises and marginal average cost declines due to the employment of one more input. In the second phase, the shape of TP becomes concave as TP starts rising at a diminishing rate due to a fall in marginal product and marginal average cost being at a constant level. These all changes in the curve of the total product occur due to the law of variable proportion.