∫(sint·cost)²dt
=∫(½·sin2t)²dt
=1/4·∫(sin2t)²dt
=1/4·∫(1-cos4t)/2 dt
=1/8·∫(1-cos4t)dt
=1/8·(t-1/4·sin4t)+C
=t/8-1/32·sin4t+C