What do the short run marginal cost, average variable cost and short run average cost curves look like?
The short run marginal cost (SMC), average variable cost (AVC), and short run average cost (SAC) curves are all U-shaped. This means these curves initially fall with increase in output, reach a minimum point, and then start rising.
The SMC curve cuts the AVC curve from below at the minimum point of AVC. To the left of this minimum point, AVC is falling and SMC is less than AVC. To the right, AVC is rising and SMC is greater than AVC.
Similarly, the SMC curve cuts the SAC curve from below at the minimum point of SAC. To the left of this minimum point, SAC is falling and SMC is less than SAC. To the right, SAC is rising and SMC is greater than SAC.
Explanation
The answer draws directly from the textbook context which explicitly states that 'Short run marginal cost, average variable cost and short run average cost curves are U-shaped.' The context also explains the relationship between SMC and the other two curves - how SMC intersects both AVC and SAC at their minimum points from below, and how SMC is less than AVC/SAC when they are falling and greater when they are rising.