Why does the short-run marginal-cost curve eventually increase for the typical firm?

The MC curve's shape is a consequence of the law of diminishing returns. Marginal cost at first declines sharply, reaches a minimum, and then rises rather abruptly. The curve's shape reflects the fact that total variable cost and therefore total cost increase at first by decreasing amounts and then by increasing amounts. For instance, if all units of a variable resource are hired at the same price, the marginal cost of each additional unit of output will fall as long as the marginal product of each additional resource is rising. Marginal cost can be found by dividing the constant cost of each additional unit of resource by the marginal product of each additional unit. But, once diminishing returns set in, the marginal product of each additional resource falls, and now when the constant cost is divided by the declining marginal product for each unit of resource, the marginal cost rises.