[bonzini]

4 and 5 too

[globnomulous]

How so? I'm too dumb to see it.

[bonzini]

The sum of the first n cubes is always the square of the sum of numbers from 1 to n. For example 1³+2³+3³+4³=(1+2+3+4)².

You can prove it by induction; just expand (n(n+1)/2)² – (n(n-1)/2)², the result is n³.