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January 31, 2015

January 31, 2015

Posted by **Isabel** on Tuesday, July 22, 2014 at 3:05pm.

Sn= 405, r=3, n=4

I don't know how to start the problem

- algebra 2 -
**Reiny**, Tuesday, July 22, 2014 at 6:13pmuse your formula for S(n)

S(n) = a(r^n - 1)/(r-1)

405 = a(3^4 - 1)/(3-1)

405 = a(81 - 1)/2

405 = 40a

a = 405/40 = 81/8

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