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# Geometric progressions watch

1. 3+6+12+......384
Find the sum
I've worked out the ratio is 2, what else do I do
2. (Original post by Lucofthewoods)
3+6+12+......384
Find the sum
I've worked out the ratio is 2, what else do I do
Yes, add them all up now.

3 + 6 + 12 + 24 + 48
3. (Original post by Naruke)
Yes, add them all up now.

3 + 6 + 12 + 24 + 48
is there not a specific formula?
4. (Original post by Lucofthewoods)
is there not a specific formula?
Yes, it's a shortcut. If you can't perform the shortcut add them all up manually.
5. (Original post by Naruke)
Yes, it's a shortcut. If you can't perform the shortcut add them all up manually.
Well what is the formula?

6. Find n by substituting 384 into the equation for a geometric series

7. (Original post by memeeee)

Find n by substituting 384 into the equation for a geometric series

Can you hint more at what to do? I'm so confused
8. (Original post by Lucofthewoods)
Can you hint more at what to do? I'm so confused
The sum is given by the formula she mentioned. The sum of the first terms of a geometric sequence with common ratio and first term is given by

You found which is correct, and you know that

The only thing left to do is find and to do that you need to use the formula for the term of a geometric sequence which is . So you would have because 384 is the last term, and solve for to see up to which term of the sequence you are summing up.

Then just substitute into the summation formula.
9. (Original post by RDKGames)
The sum is given by the formula she mentioned. The sum of the first terms of a geometric sequence with common ratio and first term is given by

You found which is correct, and you know that

The only thing left to do is find and to do that you need to use the formula for the term of a geometric sequence which is . So you would have because 384 is the last term, and solve for to see up to which term of the sequence you are summing up.

Then just substitute into the summation formula.
Thank you

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