# Math Help - Use differential to calculate the change in average cost

1. ## Use differential to calculate the change in average cost

Can anyone show me how to do this? I got the question wrong, and it is showing me the solution here. I don't know how to get this answer.

2. C(x) = (1/5) - 440/x^2

C(25)*(31-25)

C`(45)*(51-45)

C'(45) -- What's the slope at the starting point.
(51-45) -- How many horizontal units?