Question

What is the circumference of a circle whose area is 100pi " alt=" \pi " align="absmiddle" class="latex-formula">?
A) 10 B) 20 C) 10$\pi$ D) 20$\pi$E) 25$\pi$

Answer 1

$S=100\pi\\\\ \pi r^2=100\pi \ |:\pi\\\\ r^2=100 \\\\ r=\sqrt{100} \ , \ r > 0\\\\ r=10\\\\ C=2\pi r=2\pi \cdot 10=20\pi$