用点到直线距离公式得
|2sinacosa-1|/√[(sina)^2+(cosa)^2]=1/2
|sin2a-1|=1/2
sin2a-1=1/2（舍）sin2a-1=-1/2
sin2a=1/2
2a=2kπ+π/6 或 2a=2kπ+5π/6
a=kπ+π/12 或 a=kπ+5π/12