用平方差计算(x+y+1)(x+y-1),
问题描述：
用平方差计算(x+y+1)(x+y-1),
原式=[y+(x-1)][y-(x-1)]
=y²-(x-1)²
=y²-x²+2x-1
答
X^2+Xy-X+Xy+y^2-y+x+y-1 化简得x^2+2xy+y^2+y-1再化简得（x+y）^2+y+1