3 years ago

What is the sum of the geometric series in which a1 = −5, r = 3, and an = −405?

1 answer:

3 0

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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