cos(x+π/4)=√2/2(cosx-sinx)=3/5 cosx-sinx=3√2/5
(cosx-sinx)^2=(3√2/5)^2 1-2sinxcosx=18/25 sin2x=7/25
cos(x+π/4)=3/5,π/2＜x＜3π/2 x+π/4 位于第4象限 所以5π/2＜2x＜3π
cos2x=-√(1-(7/25)^2)=-24/25
sin(2x+π/4)=sin2x*cosπ/4+cos2x*sinπ/4=√2/2*(7/25 -24/25)=-17√2/50
已知cos(x+π/4)=3/5,π/2＜x＜3π/2,求sin(2x+π/4)的值
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