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# Find the sum of each of these geometric series watch

1. a) 3+6+12+...+384

I've done a=3, r=2.

I wasn't really sure where to go from there.

I tried using the formula .
EDIT- can't workout how to make the latex work. It should be the formula for the sum of the first n terms of a GP..

I got . Don't reckon that was right.
2. (Original post by Kash:))
a) 3+6+12+...+384

I've done a=3, r=2.

I wasn't really sure where to go from there.

I tried using the formula .
EDIT- can't workout how to make the latex work. It should be the formula for the sum of the first n terms of a GP..

I got . Don't reckon that was right.
Well, you need to determine n, as well.
You know that 384 is the nth element, so 384 = a*r^(n-1)
3. (Original post by FieldLeftBlank)
Well, you need to determine n, as well.
You know that 384 is the nth element, so 384 = a*r^(n-1)
How do I work out n from that?
4. (Original post by Kash:))
How do I work out n from that?
Well, you've already found that a=3 and r=2, so you have 384 = 3 * 2^(n-1)
=> 2^(n-1) = 128 = 2^7 => n-1=7

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