cos(x-a)=1/2
sin(x-a)=±√3/2
sin(x+a)+2sin2a=sin(x-a+2a)+2sin2a=cos(x-a)sin2a+sin(x-a)cos2a+2sin2a
=5/2sin2a±√3/2cos2a=0
tan2a=±√3/5