两边同时乘以√2/2
sinacos(π/4)+cosasin(π/4)=1/2=sin(π/6)
sin(a+π/4)=sin(π/6)
则a+π/4 = π±π/6+2nπ
根据a是三角形内的角,小于π大于0,得到
a = 5π/6-π/4 = 7π/12