Sequence

Here are the formulae. Photo from Google.

Sequence often appears in the last question of DSE. 

Here is 2013 Q19.

For students aiming at 5*/5**, you should understand all parts.

For students aiming at 5, you should understand a and b parts.

For students aiming at 4, you should understand a part and better try b part.

I suggest you download the photos and look through each step carefully and see the remarks made by red and blue pens.