x-y=3 7-x+y-y平方+X平方
问题描述：
x-y=3 7-x+y-y平方+X平方
已知x-y=3 求代数式7-X+y-(y-x)²的值
答
x-y=3
y-x=-3
(y-x)^2=9
7-X+y-(y-x)²=7-3-9=-5