Average Fixed Cost (AFC): AFC is the fixed cost per unit of output.
AFC = TFC/y
Since the TFC is constant throughout the short run, as y increases AFC will decline. Therefore, the AFC curve is downward sloping.
Average Variable Cost (AVC) : AVC is the variable cost per unit of output.
AVC = TVC/y.
AVC will generally decrease as the output increases. But because of the operation of the law of diminishing marginal product, the AVC will rise after a certain point. Notice that is it a mirror image of the average product curve. Manipulating the formulae of both will prove that AVC is inversely related to AP.