（sinx）^4的原函数
=1/4∫{[1-cos2x]^2}d(x)
=1/4∫(1-2cos2x+cos^2 2x) dx
=1/4∫dx - 1/4∫cos2x d(2x) + 1/4 ∫(cosx)^2 2xdx
=3/8x-1/4sin2x+1/32sin4x+C