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# Geometric series: sum to infinity watch

1. (Original post by Simplicity)
No that is wrong. When did it say n=0. I'm pretty sure it starts at n=1 and goes on.

Again, your wrong it works fine without replacing n with n-1.
It didnt say it starts with anything, even if it starts with 1 it still doesnt give you the series does it?
2. (Original post by ShortRef)
Can we please go back and actually discuss if the question is right or not.

"Sum the series 1 + 4x + 7x² + 10x³ + ... + (3N - 2)x^N."

so the general term is (3N - 2)x^N

Putting N = 0 gives -2
Putting N = 1 gives (3 - 2)x^1 = x
Putting N = 2 gives (6 - 2)x^2 = 4x^2
Putting N = 2 gives (9 - 2)x^3 = 7x^3

So the series has to be (3n - 2)x^(n-1)
N=3. Typo. n=0 gives 1

Okay that looks correct.

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Updated: August 6, 2009
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