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real formula for sum of terms of a geometric sequence

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Junior | Next Rank: 30 Posts
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real formula for sum of terms of a geometric sequence

by kjallow » Mon Mar 12, 2012 8:36 pm
what really is the formula for finding the sum of terms in a geometric/arithmetic sequence. i have seen some posts that say:

sum of terms = a(1-r^n)/(1-r) (where a is first term, r is ratio and n is # of terms)

and others that say:

sum of terms = a(1-r^n)(1-r) (where a is first term, r is ratio and n is # of terms)

Please help clarify this for me..am really confused. googling it shows the first one as the correct one. Just want to confirm.

Thanks.
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by Anurag@Gurome » Mon Mar 12, 2012 8:42 pm
kjallow wrote:what really is the formula for finding the sum of terms in a geometric/arithmetic sequence. i have seen some posts that say:

sum of terms = a(1-r^n)/(1-r) (where a is first term, r is ratio and n is # of terms)

and others that say:

sum of terms = a(1-r^n)(1-r) (where a is first term, r is ratio and n is # of terms)

Please help clarify this for me..am really confused. googling it shows the first one as the correct one. Just want to confirm.

Thanks.
That's right, it's the first one.

A geometric series looks like: {a, ar, ar2, ar3, ... }, where a is the first term and r is teh common ratio.

Sum of Geometric Series:

a + ar + ar^2 + ... + ar^(n-1) = a(1 - r^n)/(1 - r), where n is the number of terms.
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by kjallow » Mon Mar 12, 2012 9:18 pm
Thanks a bunch!!!
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by Anurag@Gurome » Mon Mar 12, 2012 9:26 pm
kjallow wrote:Thanks a bunch!!!
You are welcome.
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