sin(x－3π/4) cos(x - π/4) = - 1/4,求cos4x 将要用到：2sinAcosB = sin(A+B) + sin(A-B) 和 cos2α = cos²α - sin²α = 1 - 2sin²α sin(x－3π/4) cos(x - π/4) = 1/2 [sin(2x -π) + sin(-π/...