Maximizing Marginal Cost

[nycmath]

Precalculus
Michael Sullivan
Edition 10
Chapter 3, Section 3.3

88.

The marginal cost C (in dollars) of manufacturing x cells phones (in thousands) is given by

C(x) = 5x^2 - 200x + 4000

A. How many cell phones should be manufactured to minimize the marginal cost?

My answer is 20,000 cell phones. You say?

B. What is the minimum marginal cost?

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See attachment

My answer to part B is $2000. You say?