involving summation of a sequence

[lclcl]

I have no idea how to do question (c). Anyone can help thanks.

 

[Deleted member 4993]

View attachment 25692
I have no idea how to do question (c). Anyone can help thanks.

If I were to do this problem - I would try to make the given sequence "look like" the sequence in (a). I would note:

3 * 3 = 2 * 3 + 3

5 * 3² = 3 * 3² + 2 * 3² ........ and so on

 

[lclcl]

Thank you very much for your reply. But then how can I use 1+3+3^2...+3^(n-1)=(3^n-1)/2?

 

[lex]

If completely stuck, just write out what you know and what you want. Then look again.