a<sup>3</sup> + b<sup>3</sup>=(a+b)(a<sup>2</sup>-ab-b<sup>2</sup>)
<img src="/static/upload/39/39d3501a-cd0c-11e4-b975-00163e022551.png" />
<img src="/static/upload/6e/6e6436aa-cd0c-11e4-b975-00163e022551.png" />
正整数范围中
<img src="/static/upload/b1/b146c1a4-cd0c-11e4-b975-00163e022551.png" />
注：可用数学归纳法证明