方程这个怎么拆（x-y）的平方
(x+y)\u5e73\u65b9\u600e\u4e48\u62c6\u5206\u5f00(x+y)²
=(x+y)(x+y)
=x²+xy+xy+y²
=x²+2xy+y²
x+y=x^2+2xy+y^2-3xy
=(x+y)^2-3xy (*)
>=(x+y)^2-3(x+y)^2/4
=(x+y)^2/4
>=0
\u6240\u4ee54>=x+y
\u53730<=x+y<=4
\u82e5x+y=0\uff0c\u52190=-3xy\uff08\u7531(*)\uff09\uff0c\u6240\u4ee5x=y=0
\u82e5x+y=1\uff0c\u52191=1-3xy,xy=0\uff0c\u6240\u4ee5(x,y)=(0,1)(1,0)
\u82e5x+y=2\uff0c\u52192=4-3xy,xy=2/3\uff0c\u77db\u76fe
\u82e5x+y=3\uff0c\u52193=9-3xy,xy=2\uff0c\u6240\u4ee5(x,y)=(1,2)(2,1)
\u82e5x+y=4\uff0c\u52194=16-3xy,xy=4\uff0c\u6240\u4ee5(x,y)=(2,2)
\u7efc\u4e0a\uff0c(x,y)=(0,0)(0,1)(1,0)(1,2)(2,1)(2,2)
这是完全平方差公式，记住就行了，如果非要步骤的话，那就是两个括号中的单项式两两相乘，即
（x-y）^2=（x-y）（x-y）=x*x-x*y-y*x+y*y=x^2-2xy+y^2
(x-y)的平方 =(x-y)(x-y)=x2-2xy+y2
X的平方-Y的平方
X²－XY－Y²
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