1/（2sinacosa+cosa^2a）=(sin^2a+cos^2a)/（2sinacosa+cosa^2a）=[(sin^2a+cos^2a)/cos^2a]/[（2sinacosa+cosa^2a）/cos^2a]=(tan^2a+1)/(2tana+1)=[(-1/3)^2+1]/[2*(-1/3)+1]=10/3