降次∫sin^4θdθ=∫(sin²θ)²dθ=∫(1-cos2θ)²/4dθ=1/4∫(1-2cos2θ+cos²2θ)dθ=1/4∫dθ-1/4∫cos2θd(2θ)+1/4∫(1+cos4θ)/2dθ=1/4θ-1/4sin2θ+1/8∫dθ+1/32∫cos4θd(4θ)=3/8θ-1/4...