I had questions on 2 Problems in the Text: 1. The total cost C of producing x units of some item is a function of x. Economists use the term marginal cost for the rate of change of C with respect to x. Suppose that: C = 5x^2 + 15x + 200 What is the marginal cost when x = 15? Would this marginal cost be the cost of the 16th unit? (((I understand how to take the derivative and find dC/dx. However, I am unsure as to why this represents to cost of the 16th unit. Also, can someone explain to me in simple terms what the derivative of C represents?))) 2. Using the definition of marginal cost in the preceding exercise, suppose that the cost C of producing x units of a toy is C = 3x^2 - 4x + 5. What is the marginal cost at any value of x? Would the marginal cost necessarily increase with x in any realistic situation? (((Why doesn't the marginal cost always increase with x in any realistic situation?))) Thanks!