(cosa+π/4)=3/5,π/2≤a≤3π,所以可得a在第三象限内=cosacosπ/4-sinasinπ/4=√2/2(cosa-sina)=3/5①(sina)^2+(cosa)^2=1②5π/4＜a≤3π/2 可解得sina=-7/5√2= -7√2/10,cosa= -√2/10cos(a+π/4)=3/5,sin(a+π...
