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

1 Answer
Don't Memorise
Jun 24, 2015

color(blue)(= 50580

Explanation:

The formula for the sum of an arithmetic sequence when a, d and n are given is:

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

Here : a1 = 5, d=7 and n=120

= (120/2)[2 . 5 + (120 - 1) . 7]

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

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