There are two types of costs associated with production – Fixed Cost and Variable Cost. In the short-run, at least one factor of production is fixed, so firms face both fixed and variable cost. The shape of the cost curves in the short run reflect the law of diminishing returns.
Type of Cost
Fixed Cost - costs that do not vary with different levels of production and exist even if output is zero. Ex: rent
Average Fixed Cost - Fixed Costs/Quantity.
Average Total Cost - Total Cost/Quantity. (Total Cost = Fixed Cost + Variable Cost)
Average Variable Cost - Variable Costs/Quantity.
Note: If average costs are falling then marginal costs must be less than average while if average costs are rising then marginal must be more than average. Marginal cost on its way up must cut the cost curve at its minimum point.
MC < AV, then AC ↓
MC > AC, then AC ↑
MC = AC, AC will be at minimum