Maths formulae (A/S level)

OK, maths exam is tomorrow and I've lost a formulae! Argh!!!

I'm running through a practice paper ( http://www.aqa.org.uk/qual/gceasa/qp-ms/AQA-MPC2-W-MS-Jun05.pdf )

If you scroll down to page 3, question 3(c), it's the bloody sigma notation. They've used a formula, but I can't find this anywhere!

Any ideas?

 

What was the question? The link is just to solutions.

 

If you mean what is the sum of an Arithmetic prgression then this link should help http://en.wikipedia.org/wiki/Arithmetic_progression

 

Well, I'm certain to have failed that exam