====== 유용한 공식 ======
===== 001 =====
 $$ \sum_{x=1}^{n}x=\frac{n(n+1)}{2} $$

 $$ \sum_{x=1}^{n}x^{2}=\frac{n(n+1)(2n+1)}{6} $$

 $$ \sum_{x=1}^{n}x^{3}=\frac{n^{2}(n+1)^{2}}{4} $$

 $$ \sum_{x=1}^{n}x^{4}=\frac{n(n+1)(2n+1)(3n^{2}+3n-1)}{30} $$