## Finding an expression for the sum of n tems of the series $1^2 + 2^2 + 3^2 + … + n^2$ February 7

I know that if you have a non-arithmetic or geometric progression, you can find a sum $S$ of a series with the formula $S=f(n+1)-f(1)$ where the term $u_n$ is $u_n=f(n+1)-f(n)$. Then you can prove that with induction.What I don't understand is how I