See Matthew Fan's answer to Amy Ang's Junior College 1 H2 Maths Singapore question

3 Upvotes

0 Downvotes

The key is to realise u_n is an arithmetic progression

4 years ago

First part actually no need to find un. Just take SN - S4

N(N+14)/3 - 4(4+14)/3

= (N² + 14N)/3 - 24

Since the sum formula already given

Matthew Fan

4 years ago

That is right, yes the solution can be made much shorter that way

Amy Ang

4 years ago

thanks!