(a) A company makes computer chips from square wafers of silicon. It wants to keep the side length of a wafer very close to 15 mm, and it wants to know how the area $A(x)$ of a wafer changes when the side length $x$ changes.
Find $A'(15)$. [Can you explain its meaning in this situation?]

(b) What is the rate of change of the area of the square with respect to its side length when the perimeter is $26$ mm?

(a) $A'(15)=$ sq mm/ mm
(b) $A'(?)=$ sq mm/mm