利用公式cos2x = cos(x+x) = cosxcosx - sinxsinx = (cosx)^2 - (sinx)^2 = 2(cosx)^2 -1
1/4(cos2x)^2+3/4
=1/4 * (1+cos4x)/2 +3/4
=1/8(1+cos4x)+3/4