若X2 +y2 -4x-6y= -13 求分式x2-2y2/2xy的值
(x^2-4x+4)+(y^2-6y+9)=0
(x-2)^2+(y-3)^2=0
x-2=0
y-3=0
x=2
y=3
x2-2y2/2xy=(2^2-2*3^2)/(2*2*3)=-7/6