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

3 answers

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
why didn't you use this equation: 765=3(2)^n-1
wait I understand thanks for your help :D