sinα=-2cosα
sin^2α+cos^2α=1
5cos^2α=1
cosα=±√5/5,sina=±2√5/5
①cosα=√5/5,sinα=-2√5/5
②cosα=-√5/5,sinα=2√5/5
sin2α=2sinαcosα=-4/5
①sin2α+2cosα=-4/5+√5/5=（√5-4）/5
②sin2α+2cosα=-4/5-√5/5=-（√5+4）/5