y = (sinx)^4+(cosx)^4 = [(sinx)^2+(cosx)^2]^2-2(sinx)^2(cosx)^2 = 1-(1/2)(sin2x)^2 = 1-(1/4)(1-cos4x) = 3/4+(1/4)cos4x.y' = -sin4x, y''=-4cos4x, y'''=16sin4x, ......,y^(n) = 4^(n-1)cos(4x+nπ/2)
