Question

Find the circumference of a circle whose area is 4 times the area of circle with diameter 21cm.

Answer 1

let the radius be r cm
$\pi \: r {}^{2} = 4\pi \: r {}^{2}$
r=21cm
$2\pi \: r \: = \: 2 \times 22 \div 7 \times 21 \\ 2 \times 22 \times 3 \: = 132cm$