cos(2x+60°)=cos2xcos60-sin2xsin60=-12/13
sin(2x+60°)=sin2xcos60+cos2xsin60=-5/13
cos(2x+60°)=cos2xcos60-sin2xsin60=-12/13
sin(2x+60°)=sin2xcos60+cos2xsin60=-5/13
sin2xcos60+cos2xsin60=-5/13
cos60=1/2 sin60=√ 3/2
带进去。1/2cos2x-√ 3/2sin2x=-12/13
1/2sin2x+√ 3/2cos2x=-5/13
然后解出sin2x 和cos2x