sin^2α-2cos^2α+1=sin^2-2cos^2+sin^2a+cos^2a=2sin^2-cos^2a=(2sin^2-cos^2a)/1=(2sin^2-cos^2a)/(sin^2a+cos^2a)同除cos^2a=（2tan^2a-1)/(tan^2a+1)=(8-1)/(4+1)=7/5