cos(x+π/4)=√2/2(cosx-sinx)=3/5 cosx-sinx=3√2/5(cosx-sinx)^2=(3√2/5)^21-2sinxcosx=18/25sin2x=7/25cos(x+π/4)=3/5,π/2＜x＜3π/2x+π/4 位于第4象限 所以5π/2＜2x＜3πcos2x...