How do you find $S_{n}$ $S_n$ for the arithmetic series given $a_{1} = 7$ $a_1=7$ , d=-2, n=9?

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Answer 1 · 2017-01-27T17:50:01

$S_{n} = - 9$ $S_n=-9$

This is an arithmetic series whose first term is $7$ $7$ and common difference is $- 2$ $-2$ i.e. series is $7,5,3,1,-1,....$

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

we have $S_9=9/2(2xx7+(9-1)(-2))$

= $9/2xx(14-8xx2)=9/2xx(-2)=-9$

How do you find S_nSnS_n for the arithmetic series given a_1=7a1=7a_1=7, d=-2, n=9?