f(a)=sin^2acosatana/(-sina)(-tana)=sinacosaf(a)=(1/8)(cosa-sina)^2=cos^2a+sin^2a-2sinacosa=1-2*(1/8)=3/4a∈(π/4,π/2)∴cosa∴cosa-sina<0∴cosa-sina=-√3/2
