SOLUTION: Deret geometri 2+2^2+2^3+....+2^n=254, n=....?

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Question 398083: Deret geometri 2+2^2+2^3+....+2^n=254, n=....?
Answer by Tatiana_Stebko(1539) About Me  (Show Source):
You can put this solution on YOUR website!
sum of the n numbers in a geometric progression
S=%28a%28r%5En-1%29%29%2F%28r-1%29
a=2, r=2
S=254
%282%282%5En-1%29%29%2F%282-1%29=254
2%5En-1=127
2%5En=128
2%5En=2%5E7
n=7