>424

〔逆函数定理〕
y=f(x) の逆函数を x=g(y) とすると
　f'(x)g'(y) = 1.

(左辺) = f '(x){1 + g'(y)sin(y)},

　x = g(y) = ∫g'(y)dy = -∫dy/sin(y) = -∫dt/t = -log|t| +c = -log|tan(y/2)| +c,

ここに t = tan(y/2).