Calculus
posted by THU .
The cost (in dollars) of producing x units of a certain commodity is c(x)=5000+10x+0.05x^2
Find A=100 to x=105
so far i got
yy/xx= C(105)C(100)/105105
i don't understand how it comes out to be 101.25/5
please help me

You have not said what A is. You also introduce y without definition. You need to add parentheses to clarify your yy/xx equation.
Please restate the problem exactly as it was presented to you. 
I appears that you are attempting to calculate the average rate of change of the cost as the number of units changes from 100 to 105
that rate would be [C(105)C(100)]/[105100]
which does work out to be 101.25/5 
wait so whats is c(105) is it 5000+10x+0.05x^2 times 105?

It appears you are not familiar with the function notation.
In short
if C(x) = 5000+10x+0.05x^2
then C(105) = 5000 + 10(105) + .05(105)^2
(whenever an x appeared, we replaced it with 105, the same thing with C(100) )