cos2a=-23/25=2cos^2a-1 ,cos^2a=1/25 ,cosa=-1/5
cosa=2cos^2(a/2)-1=-1/5 ,cos^2(a/2)=2/5 ,cosa/2=±√10/5 稍等5/2π＜a＜3π , 5π/4＜a/2＜3π/2 ,cosa/2=-√10/5sina/2=-√[1-(cosa/2)^2]=-√15/5tana/2]=(-√15/5)/(-√10/5)=√6/2