tana/（tana-1）=-1,tana=1/2sin²a+sinacosa+2=(sin²a+sinacosa)/1+2=(sin²a+sinacosa)/(sin^2a+cos^2a)+2 （上下同除以cos^2a)=(tan^2a+tana)/(tan^2a+1)+2=3/5+2=13/5