An Equation With Squares

Let k ≥ 0, and let n = k(2k + 1). Then we have:

n² + (n+1)² + ... + (n+k)² = (n+k+1)² + ... + (n+2k)².

For example,
0² = 0,
3² + 4² = 5²,
10² + 11² + 12² = 13² + 14²,
21² + 22² + 23² + 24² = 25² + 26² + 27²,
and so on.