# Math Help - Sigma Notation Closed and Open Forms

1. ## Sigma Notation Closed and Open Forms

k=1
ending value of k=n-1
eqn= k^3/n^2

How do you deal with the (n-1) ending value when you are writing the sum in closed form?

2. Originally Posted by thinair
k=1
ending value of k=n-1
eqn= k^3/n^2

How do you deal with the (n-1) ending value when you are writing the sum in closed form?
You should know that $\sum_{k=1}^n k^3 = \frac{n^2 (n+1)^2}{4}$.

Therefore $\sum_{k=1}^{n-1} k^3 = \frac{(n-1)^2 n^2}{4}$.

3. Thank you, that is much simpler than what I was trying.