You don't need to evaluate (n(n+1)/2)2

Given these summation identities:

1 + 2 + 3 + ... + n = n(n+1)/2

13 + 23 + 33 + ... + n3 = n2(n+1)2/4 = (n(n+1)/2)2

it is easy to show that

13 + 23 + 33 + ... + n3 = (1 + 2 + 3 + ... + n)2