已知cos(π/4-x)=-4/5,5π/4我看不到啊回答在哪呢

∵5π/4＜x＜7π/4，则-3π/2又∵cos(π/4-x)=-4/5，则(cosx+sinx)/√2=-4/5，即sinx+cosx=-4√2/5
两边平方，得2sinxcosx=7/25
∴sin(π/4-x)=-3/5，tan(π/4-x)=3/4
∴原式=(2sinxcosx-2sin²x)/(1+tanx)
=2sinx(cosx-sinx)/(1+sinx/cosx)
=2sinxcosx(cosx-sinx)/(sinx+cosx)
=2sinxcosx(1-tanx)/(1+tanx)
=2sinxcosxtan(π/4-x)
=7/25×3/4=21/100

∵ cos(π/4-x)=-4/5,∴2/√2cosx+2/√2sinx=-4/5,得sinx+cosx=-4√2/5.①
﹙sinx+cosx﹚²=32/25, 得1+2sinxcosx=32/25,得2sinxcosx=sin2x=7/25.②
又5π/4