(x+y)(x的2次-xy-y的2次)-(x-y)(x的2次+xy+y的2次)

(x+y)(x^2-xy-y^2)-(x-y)(x^2+xy+y^2)
=(x^3+y^3)-(x^3-y^3)
=2y^3

这就是简便运算了
所谓简便运算是把原本复杂的东西简单化计算。
已经达到目的了。

(x+y)(x^2-xy-y^2)-(x-y)(x^2+xy+y^2)
x^2-xy-y^2，y^2减号改为加号，才能简便运算。
=(x^3+y^3)-(x^3-y^3)
=2y^3

(x+y)(x^2-xy-y^2)-(x-y)(x^2+xy+y^2)
=(x^3+y^3)-(x^3-y^3)
=2y^3