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**Truthbetold** Suppose that the dollar cost of producing x washing machines is $\displaystyle c(x)= 2000 + 100x -.1x^2$

Simplified it is: $\displaystyle c(x)= x^2 - 1000x -20000$

(a) Find the average cost of producing 100 washing machines.

Don't get it or what to use.

(b) Find the marginal cost when 100 machines are produced.

I know how to do this one, Taek the derivative and plug in x.

(c) Show the marginal cost when 100 washing machines are produced is approximately the cost of producing one more washing machine after the first 100 have been made, by calculating the latter cost directly.

Don't understand what they want.

For the average cost, I think you take the derivative of it and then plug x= 100.

However, I got a negative number.

In the book, it says " [the difference quotient without the limit] = the average cost of each of the additional *h* tons produced.

So, do I plug it in and just deal with the highly messy equation or am I missing something?