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What is the sum of the geometric series in which a1 = −5, r = 3, and an = −405?
A hint on how to solve is attached!
r ≠ 1, where a1 is the first term and r is the common ratio.

What is the sum of the geometric series in which a1 5 r 3 and an 405 A hint on how to solve is attached r 1 where a1 is the first term and r is the common ratio class=

Respuesta :

an = a1 * r^(n-1)

-405 = -5*3^(n-1)

3^(n-1) = 81

3^4 = 81  so n-1 = 3  and n =  4

Sn  = S4  =  -5 (1 - 3^4)   / ( 1 - 3)   =  400 / -2 = -200 Answer

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