繁根式化简

形如\sqrt{a \pm \sqrt{b} } ，定义\Delta = a^2-b，若有\sqrt{ \Delta } \in N^* ，则有

\sqrt{a \pm \sqrt{b} } =\sqrt{ \frac{a+\Delta}{2} } \pm \sqrt{ \frac{a-\Delta}{2} }

多元完全平方展开

( \sum_{i=1}^{n} a_{i} )^2 - ( \sum_{i=1}^{n} a_{i}^2 ) = a_{1}a_{2}+a_{1}a_{3}+a_{1}a_{4}+...+a_{1}a_{n}+a_{2}a_{3}+a_{2}a_{4}+...+a_{n-1}a_{n}