# Using differentials find the approximate decrease on the area of a circle when the radius of it decreases from $r = 1 cm$ to $r = 0.8 cm$

I am asked to solve the following problem:

Using differentials find the approximate decrease on the area of a circle when the radius of it decreases from $r = 1 cm$ to $r = 0.8 cm$

What I have so far is

$$r' = -0.2 cm\\ \\ A' = 2 \pi r \cdot r'\\ A' = 2 \pi \cdot 1 \cdot (-0.2)\\ A' = -0.4 \pi cm^2$$

Is that correct? Am I missing something here?

Thank you.

I would use primes for derivatives and $\Delta$ for changes, so $A'=2\pi r, \Delta A = 2 \pi r \Delta r$ and so on. The answer and approach are fine.