第5章 5.11

$r = a\sin\theta \qquad (0 \le \theta \le \pi, a > 0)$

$S = \int_0^\pi \sqrt{a^2\sin^2\theta + a^2\cos^2\theta}\,d\theta = \int_0^\pi a\,d\theta = a[\theta]_0^\pi = \pi a$