C1....chapter 6? Watch
(You could be asked to prove these formula) What does it mean
its the formula for the sum of an arithmetic series
if it does ask you to derive but not directly telling you, just spot it and then derive
It means that you could be asked to show, for example, that the th term of an arithmetic sequence with initial term and common difference is , or that the th partial sum is , etc... that is, you might have to derive these results rather than quoting them.
They're the same, so I doubt it matters I find writing it as easier to remember (and easier to derive), since it emphasises that you've got n lots of the initial term and (1+2+...+n-1) lots of the common difference. The factorised form sort of garbles this information.
Then reverse it:
Sn = (a+(n-1)d) +..................+a
When you add the 2 lines together you get
2Sn = (2a+(n-1)d)+(2a+(n-1)d)..... (n times)
Then divide by 2
Sn = 0.5n(.....)