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