sin^2 80 + sin^2 40 - cos50cos10
=cos^2 10 + sin^2 40 - cos50cos10
=(1/2)[(1+cos20)+(1-cos80)-(cos60+cos40)]
=(1/2)[3/2+cos20-(cos80+cos40)]
=(1/2)[3/2+cos20-2cos60cos20]
=3/4