sin(2x-π/4)=sin2xcosπ/4-cos2xsinπ/4=根号2/2(sin2x-cos2x)=根号2/2(2sinxcosx-(cosx)^2+(sinx)^2)
因为x属于(派,3/2派) 所以cosx=-2根号5/5,代入上式就可以了