因为sin^2a+cos^2a=1所以sin^2a-sinacosa+cos^2a=(sin^2a-sinacosa+cos^2a)/(sin^2a+cos^2a)上下同除cos^2a=(tan^2a-tana+1)/(tan^2a+1)=1/2tan^2a+1=2tan^2a-2tana+2tan^2a-2tana+1=0tana=1