∑ n = 1 N n 3 = N 2 ( N + 1 ) 2 4 = ( ∑ n = 1 N n ) 2 {\displaystyle \sum _{n=1}^{N}n^{3}={\frac {N^{2}(N+1)^{2}}{4}}=\left(\sum _{n=1}^{N}n\right)^{2}}
