How do you find d for the arithmetic series with s17=-170 and a1=2?

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Answer 1 · 2016-08-25T02:10:44

$d = -1.5$

We use the formula $s_n = n/2(2a + (n - 1)d))$ to find the sum of an arithmetic series.

In our problem, we have that $n = 17$ , $s_17 = -170$ and $a = 2$ . We are looking for $d$ .

$-170 = 17/2(2(2) + (17 - 1)d)$

$-170 = 17/2(4 + 16d)$

$-170 = 34 + 136d$

$-204 = 136d$

$d = -1.5$

Hopefully this helps!