cos(派/2+a)-2sin(a-派/2)=0 sin(-a)-2cos(π-a)=0-sina+2cosa=0sina=2cosatana=2sin^2a+sinacosa-2cos^2a=(sin^2a+sinacosa-2cos^2a)/(sin^2a+cos^2a)=(tan^2a+tana-2)/(tan^2a+1)=(4+2-2)/(4+1)=4/5