x^2-7xy+12y^2=0(x-3y)(x-4y)=0x=3y或x=4y当x=3y时 (x^2-2xy+y^2)/2xy=(x-y)^2/2xy=(3y-y)^2/2*3y*y=4y^2/6y^2=4/6=2/3当x=4y时 (x^2-2xy+y^2)/2xy=(x-y)^2/2xy=(4y-y)^2/2*4y*y=9y^2/8y^2=9/8