∫(sinx)^4dx
=∫[(sinx)^2]^2dx
=∫1/4(1-cos2x)^2dx
=∫1/4[1-2cos2x+(cos2x)^2]dx
=∫1/4[1-2cos2x+(1+cos4x)/2]dx
=∫(3/8-1/2cos2x+1/8cos4x)dx
=3/8x-1/4sin2x+1/32sin4x+C