Hello !

The idea is good ! (I wonder how you got it)

however, I can point out two problems :

In your second line, you forgot the part, but I guess it is just a typo.

When substituting different values of , you wrote 6(1), 6(2)... which is false since it is4i in the formula. Moreover, you forgot each time to add 1.

So adding them together shall give something like :

Edit : and yes, your expression for the sum of the squares is correct =)