sin(x- 3π/4)
=sin(x-π/4-π/2)
=-sin[π/2-(x-π/4)]
=-cos(x-π/4)
所以由已知,
-cos（x- π/4）^2=—1/4
所以cos(x-π/4)^2=1/4
所以cos2(x-π/4)=2cos(x-π/4)^2-1
=-1/2
所以
cos4x
=-cos4(x-π/4)
=-[2cos2(x-π/4)^2-1]
=-[2*(-1/2)^2-1]
=1/2
好好学习,天天向上,呵呵!