ax^2+2xy-x-(3x^2-1\2 bxy+3y)=(a-3)x^2+(2-b/2)xy-x-3y不含二次项,
于是a-3=0,2-b/2=0,所以
a=3,b=4,
于是a^2-2b=3^2-2X4=9-8=1