Hi guys,

I have a math assignment here dealing with geometric series/sequences, arithmetic series/sequences, and the recursion formula. The problem is that I don't know which formula applies to which question!

If you could, a detailed explanation would be really wonderful.

Here are the questions:

1. Find $\displaystyle tn$ and $\displaystyle t11$ for the sequence $\displaystyle 6x,12x^2, 24^3...$

(For this one, I use the geometric sequence, right?)

2. Find $\displaystyle s31$ for $\displaystyle 36+34+32+...$

3. Each year, for 8 years, the population of a city increased by 3.5% of its value in the previous year. If the initial population was 320 000, what was its population by the end of the 8th year?

4. If a sequence is determined by $\displaystyle f(x) = 6^x-1$ where $\displaystyle x$ is a natural number, find the sum of the first 15 terms.

Of course I don't expect anyone to help me with all of them, but even if you helped me figure out which formula to use for any of them, I would be really grateful!

Thank you so much in advance!