(x-y)的平方+2x-2y+1把x-y看成整体得(x-y)的平方+(2x-2y)+1得(x-y+1)的平方
(x-y)^2+2x-2y+1=(x-y)^2+2(x-y)+1把x-y看成整体原式=(x-y+1)^2
x^2+y^2-2xy+2x-2y+1