若cos(π/4+x)＝3/5,17/12π＜x＜7/4π,求(sin2x+2sinx)/(1-tanx)的值
问题描述：
若cos(π/4+x)＝3/5,17/12π＜x＜7/4π,求(sin2x+2sinx)/(1-tanx)的值
答
17π/12＜x＜7π/4,得5π/3＜x+π/4＜2π cos(x-π/4)=cos[(x+π/4)-π/2]=sin(x+π/4)=-√[1-cos²(x+π/4)]=-√[1-(3/5)²]=-4/5 sin(2x)=-cos(2x+π/2)=-cos[2(x+π/4)]=1-2cos²(x+π/4)=1-2•...