已知cos(a+π/4）=3/5,x/2
问题描述：
已知cos(a+π/4）=3/5,x/2
答
π/2≤a≤3π/2,cos(a+π/4）=3/5,就是在第四象限；3π/2＜a+π/4＜2π,3π＜2a+π/2＜4π,π＜2a+π/2＜2π,3π-π/4＜2a+π/4＜4π-π/4,cos(a+π/4）=3/5,sin(a+π/4）= -4/5,cos(2a+π/2)=9/25-16/25=-7/25;sin(...