The short-run average total cost curve is 
-shaped because at low output levels the spreading effect of falling average fixed costs dominates the diminishing returns effect, while at high output levels the reverse is true.
Definitions:
Production Function
An equation that specifies the amount of product that can be produced from different quantities of inputs.
Returns To Scale
The rate at which production output increases as a result of a proportional increase in all input factors.
Marginal Products
The additional output that is produced by employing one more unit of a particular input while holding the quantities of all other inputs constant.
Exponents
Mathematical notations indicating the number of times a base number is multiplied by itself.
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