sin2a/sin2b=3
sin(a+b+a-b)/sin(a+b-a+b)=3
{sin(a+b)cos(a-b)+cos(a+b)sin(a-b)}/{sin(a+b)cos(a-b)-cos(a+b)sin(a-b)}=3
sin(a+b)cos(a-b)+cos(a+b)sin(a-b)=3{sin(a+b)cos(a-b)-cos(a+b)sin(a-b)}
4cos(a+b)sin(a-b)=2sin(a+b)cos(a-b)
tan(a-b)/tan(a+b)=1/2