The joint cost (in dollars) for products X and Y is given by C(x, y) = 40

Question:

The joint cost (in dollars) for products X and Y is given by
C(x, y) = 40 + 3x2 + y2 + xy
where x represents the quantity of X and y represents the quantity of Y.
(a) Find and interpret the marginal cost with respect to x if 20 units of product X and 15 units of product Y are produced.
(b) Find and interpret the marginal cost with respect to y if 20 units of X and 15 units of Y are produced.