x2 – 2xy + 3y2 – 2=(x-y)^2-2+2y=(x-y)^2-2(1+y^2)=(x-y)^2-2(1+y)(1-y)因为 y= x – 1 代入所以式子变为(x-x+1)^2-2(1+x-1)(1-x+1)=1-2x(2-x)=2x^2-4x+1
