Question

How do you find the indicated sum for the arithmetic series S15 for 5 + 9 + 13 + 17 + ...?

Answer 1

`S_15 = 495`

Explanation:

In this Arithmetic series : the first term a = 5 , the common difference d = 9- 5 = 4 and n = 15.

Using `color(red)( S_n = n/2 ( 2a + ( n - 1 )d )`

`rArr S_15 = 15/2 ( 2 xx 5 + (15 - 1 )xx4 )`

`rArr S_15 = 15/2 (10 + 56 ) = 495`