>>572
ヒント:
(a^2)*sin^2(x) - (x^2)*sin^2(a)
= {(a^2)*sin^2(x) - (a^2)*sin^2(a)} - {(x^2)*sin^2(a) - (a^2)*sin^2(a)}
= (a^2)*{sin^2(x) - sin^2(a)} - sin^2(a)*{(x^2) - (a^2)}