a=π/6.划图一目了然.
y=x於X轴夹角为π/4,得y=x直线与角π/6终边夹角大小为π/4-π/6=π/12;
而角2a的终边与角π/6的终边关于直线Y=x对称,即Y=x为其两终边夹角的角平分线
故得角2a=π/12+π/4=π/3,故得a=π/6.
这里不方便划图啊.