俊朗精英团队为你
20=3x^2+3y^2-xy=3x^2+3y^2-4*(1/4xy)>=3x^2+3y^2-4(x^2+1/16y^2)*1/2
=x^2+3y^2-1/8y^2=1/8(8x^2+23y^2)
8x^2+23y^2>=160