2x^2+5xy+3y^2=0
(x+y)(2x+3y)=0
x=-y或x=-3y/2
∴k=2x+3y+xy=y-y^2=-(y-1/2)²+1/4,最大值为1/4;
或者,k=2x+3y+xy=-3y²/2,最大值为0