tan(a/2)+1/tan(a/2)=tan(a/2)+cot(a/2) =sin(a/2)/cos(a/2)+cos(a/2)/sin(a/2)=[sin^2(a/2)+cos^2(a/2)]/[sin(a/2)cos(a/2)] =2/[2sin(a/2)cos(a/2) =2/sina2/sina=5/2 5sina=4 sina=4/5 由于0 0 cosa=√[1-sin^2 a...