How do you find the sum of the arithmetic sequence given a1 = 5, d = 7, n = 120?

Jun 24, 2015

 color(blue)(= 50580

Explanation:

The formula for the sum of an arithmetic sequence when $a , d \mathmr{and} n$ are given is:

color(blue)(Sum = (n/2)[2a + (n-1)d]

Here :$a 1 = 5 , d = 7 \mathmr{and} n = 120$

$= \left(\frac{120}{2}\right) \left[2 . 5 + \left(120 - 1\right) . 7\right]$

$= 60 . \left[10 + 119 . \left(7\right)\right]$
$= 60 . \left[10 + 833\right]$
$= 60 . \left(843\right)$
 color(blue)(= 50580