f(x) = (cosx)^4 - 2sixcosx - (sinx)^4= (cos?x + sin?x)(cos?x - sin?x) - sin2x= cos2x - sin2x= √2cos(2x + π/4) 【sin(π/4) = cos(π/4) = √2/2】最小正周期为T = 2π/2 = π
