We can prove this by mathematical induction......
First.....the sum of the first n positive integers = [n][n+1]^2/ 2
Therefore, the [sum of the first n positive integers]^2 = [n]^2 [n =1]^2 / 4
So....we want to prove that
1^3 + 2 ^3 + 3^3 + .....+ n^3 = [n]^2 [n =1]^2 / 4
Show that it is true for n = 1
1^3 = [1]^2 [2)^2 / 4 = [1][4]/4 = 1
Assume it's true for k.....that is......
1^3 + 2^3 + 3^3 + ....... + k^3 = [k]^2 [k + 1]^2/ 4
Show that it is true for k + 1 .... that is.........
1^3 + 2^3 + 3^3 + .......+ k ^3 + [k + 1]^3 = [k + 1]^2 [k + 2]^2 / 4
So we have.......
1^3 + 2^3 + 3^3 + .......+ k ^3 + [k + 1]^3 = [k]^2 [k + 1]^2 / 4 + [k + 1]^3
1^3 + 2^3 + 3^3 + .......+ k ^3 + [k + 1]^3 = [k + 1]^2 ( k^2 + 4[k + 1] ) / 4
1^3 + 2^3 + 3^3 + .......+ k ^3 + [k + 1]^3 = [k + 1]^2 ( k^2 + 4k + 4 ) / 4
1^3 + 2^3 + 3^3 + .......+ k ^3 + [k + 1]^3 = [k + 1]^2 [k + 2]^2 / 4
Which is what we wanted to prove
