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?