Business Economics

If the marginal cost curve is falling, then the average total cost curve must be falling too. It is because the EXTRA (marginal) cost per unit falls, thus the average cost (total cost divided by the total number of output) must be fallen too.
圖片參考：http://imgcld.yimg.com/8/n/HA00150697/o/701010310123813873408580.jpg
Therefore, Marginal cost curve must cut the lowest of average cost curve from bottom.