# How do you find the sum of Sigma 5 where k is [1,5]?

${\sum}_{\text{k=1,5}} k = 15$
The sum of the first $k$ natural numbers is given by: $\frac{k \left(k + 1\right)}{2}$
Hence: ${\sum}_{\text{k=1,5}} k = \frac{5 \left(5 + 1\right)}{2} = \frac{5 \cdot 6}{2} = 15$