The sum of the first n squares is
The numbers are called the square pyramidal numbers.
One of the simplest proofs uses induction on n. This approach assumes that you know (or guess) the correct formula beforehand, though.
This post will show a derivation which is a formalization of the derivation shown on wikipedia. It revolves around manipulating sums and the fact that
We will now write in three different ways. The first simply inserts the above expression for :
The second reverses the order of summation for the inner sum:
The third starts as the first and does a series of manipulations:
(the manipulations being: Switching the order of summation, change of variable , change of variable , renaming , ).
We now add together these three expressions for and get
which, after dividing each side by 3, produces the wanted formula.