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April 19, 2015

April 19, 2015

Posted by **Rachel** on Monday, May 7, 2012 at 1:58pm.

Sn=189

Find n for the given sum if the geometric series

- Algebra 2 -
**Reiny**, Monday, May 7, 2012 at 2:08pmlooks like

a = 5 and r = -1/2

so 5((1 - (-1/2)^n)/(1-(-1/2)) = 189

1 - (-1/2)^n = 56.7

(-1/2)^n = -56.7

since we know that n must be a whole number and for the above to be true, n must be odd

so we could solve

(1/2)^n = 56.7

using logs,

n = log 56.7/log .5 = a negative

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