# Thread: Geometric Progression with logs

1. ## Geometric Progression with logs

Firstly, I know how to find the number of terms of Un=ar^(n-1) using logs. However, I am stuck on a question using the formula; Sn=a(1-r^n)/1-r

I assumed that the process would be the same with rearanging but I am not getting correct answer.

The question is;

A post is being driven into the ground by a mechanical hammer. The distance it is driven by the first blow is 8cm. Each subsequent blow drives the post 90% of the distance of the proceeding blow. If each blow takes 10 seconds how long will it take to drive the post 90% of the distance of the proceeding blow?

2. ## Re: Geometric Progression with logs

Hello,peejaygee!

No one is answering because your problem makes no sense.
Please review the wording of the last sentence.

A post is being driven into the ground by a mechanical hammer.
The distance it is driven by the first blow is 8cm.
Each subsequent blow drives the post 90% of the distance of the proceeding blows.
If each blow takes 10 seconds, how long will it take to drive the post 90% of the distance of the proceeding blow?

Obviously, the passage in blue is incorrect.

Please supply the correct distance.

3. ## Re: Geometric Progression with logs

Originally Posted by peejaygee
Firstly, I know how to find the number of terms of Un=ar^(n-1) using logs. However, I am stuck on a question using the formula; Sn=a(1-r^n)/1-r

I assumed that the process would be the same with rearanging but I am not getting correct answer.

The question is;

A post is being driven into the ground by a mechanical hammer. The distance it is driven by the first blow is 8cm. Each subsequent blow drives the post 90% of the distance of the proceeding blows. If each blow takes 10 seconds how long will it take to drive the post 90% of the distance of the proceeding blow?
In addition to the sentence soroban marked in blue (which either makes no sense or is trivial), is it 90% of the sum of the preceding blows or 90% of the immediately preceding blow? In one place you use the plural and in another the singular.

4. ## Re: Geometric Progression with logs

That is what was written on my test paper.

5. ## Re: Geometric Progression with logs

It is 90% of the last blow.