SOLUTION: The marginal cost (C) of manufacturing x cell phones (in thousands) is given by: {{{C(x)= 5x^2-200x+4000}}} A. How many cell phones should be manufactured to minimize the marginal

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Question 1098121: The marginal cost (C) of manufacturing x cell phones (in thousands) is given by:
A. How many cell phones should be manufactured to minimize the marginal cost?
B. What is the minimum marginal cost?
My work was =
A. Using = = 20 or 20,000 phones to minimize the marginal cost.
B. I subbed in 20 as the x value in the given equation resulting in the minimum marginal cost being $2000.
I am uncertain if this is how to approach the question.

Found 3 solutions by josgarithmetic, Boreal, ikleyn:
Answer by josgarithmetic(39617)   (Show Source): You can put this solution on YOUR website!



Answer by Boreal(15235)   (Show Source): You can put this solution on YOUR website!
Yes, your approach is fine.
-b/2a is the approach to get the vertex of the minimum, and it does equal 20, or 20,000 phones.
5(20^2)-200(20)+4000=2000-4000+4000=$2000

Answer by ikleyn(52781)   (Show Source): You can put this solution on YOUR website!
.
The solution and the answer  "10"  given by @gosgarithmetic is   W R O N G.


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