# Math

How many terms of the series 3+6+12+24+...add to 765?

1. 👍 0
2. 👎 0
3. 👁 72
asked by Han
1. looks like a geometric series with

a = 3
r = 2

sum(n) = a(r^n -1)/(r-1)
765 = 3(2^n - 1)/1
255 = 2^n -1
256 = 2^n
2^8 = 2^n

n = 8

there are 8 terms

1. 👍 0
2. 👎 0
posted by Reiny
2. why didn't you use this equation: 765=3(2)^n-1

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2. 👎 0
posted by Han
3. wait I understand thanks for your help :D

1. 👍 0
2. 👎 0
posted by Han

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