tana-cosa=0
sina/cosa-cosa=0
(sina-cos^2a)/cosa=0
(sina-1+sin^2a)/cosa=0
sin^2a+sina-1=0
sin^2a+sina+1/4=1+1/4=5/4
(sina+1/2)^2=5/4
sina+1/2=±√5/2
sina=(-1±√5)/2
因为-1