One of the interesting applications of arithmetic series is the result that
$$\sum_{r=1}^n r = 1+2+3+\ldots + n = \frac{1}{2} n (n+1)$$
One of the interesting applications of arithmetic series is the result that
$$\sum_{r=1}^n r = 1+2+3+\ldots + n = \frac{1}{2} n (n+1)$$