(sinx)^4 = (1/4)[2(sinx)^2]^2 = (1/4)(1-cos2x)^2= (1/4)[1-2cos2x+(cos2x)^2]= (1/4)(1-2cos2x + 1/2+(1/2)cos4x)= (1/8)(3-4cos2x+cos4x)然后套用 cos2x， cos4x 的傅里叶级数展开公式即得。
