How do you find the sum of the arithmetic sequence having the data given $a_{1} = 5$ $a_1=5$ , d = 7, n = 120?

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Answer 1 · 2016-02-14T18:14:50

$S_120=sum_(n=1)^120 [5+(n-1)(7)] =5+12+19+26+......+838=42993$

The formula for the sum to $n$ terms of an arithmetic sequence with first terms $a$ and common difference $d$ is given by

$S_n=n/2[2a+(n-1)d]$

$therefore S_120=102/2[(2)(5)+(120-1)(7)]$

$=42993$ .

How do you find the sum of the arithmetic sequence having the data given a_1=5a1=5a_1=5, d = 7, n = 120?