The Average- Marginal Relationship Consider the line drawn from origin to point A in(a). The slope of the line measures average variable cost(a total cost of $175 divided by an output of 7, or a cost per unit of $25) Because the slope of the VC curve is the marginal cost the tangent to the VC curve A is the marginal cost of production when output is 7. At A, this marginal cost of $25 is equal to the average variable cost of $25 because average variable cost is minimized at this output.