Briefly explain why

MATHEMATICS · Subjective · AP · Calculus BC · Parametric, Polar, and Vector-Valued Functions · Parametric, Polar, and Vector-Valued Functions

Problem

Briefly explain why, when computing the exact area enclosed by the curve

r = 3\sin \theta

for

0\le \theta \le \pi

, one may integrate over

0 \le \theta \le \tfrac{\pi}{2}

and then double the result.