Math  average rate problems(check + help)
posted by Anonymous .
The total cost, c, in dollars of operating a factory that produces kitchen utensils is C(x)=0.5x^2+40x+8000, where x is the number of items produced in thousands.
a)Determine the marginal cost of producing 5000itmes and compare this with the actual cost of producing the 5001st item.
Ans: C'(5)= 5+40=45 and the comparison is C(5.001)c(5)=.045
b) The average cost is found by dividing the total cost by the number of items produced. Determine the average cost of producing 5000items.
Ans: avg cost = total cost/5000
=$1.64
c)Determine the rate of change of the average cost of producing 5000 items. Interpret this value.
Ans: i tried to finding the derivative of the avg cost but i don't get the required ans 0.32

c) (unless I am not reading this correctly)
according to the definition
avg(x) = (0.5x^2+40x+8000)/x
= .5x + 40 + 8000/x
avg '(x) = .5  8000/x^2
avg '(5000) = .5  8000/5000^2 = .4998 
x is the number of items produced in thousands