若cos(x-π/4)=-3/4,则sin2x的值为多少 ~~~~~~~~~看清楚题目~~~速度点~~~~~~~

cos(x-π/4)=-3/4,cos(x)cos(π/4）+sinxsin(π/4）=-3/4 cosx+sinx=-3根号2/4
（ cosx+sinx)^2=(-3根号2/4)^2 sin^2x+cos^2x+2sinxcosx=9/8 2sinxcosx=1/8 sin2x=2sinxcosx=1/8