You are Here: Home >< Maths

# Sequences and Series help watch

1. I know this is an easy question but once you've overanalysed something its hard to go back !
The sum of the first n numbers is sn = n/2 (2a+(n-1)d) or sn= n/2 (a+l)
And the sum of the first n whole numbers is sn= n/2 n(n+1)

My question is, what differs between these two formulas? When do you use each one? Why does the sum of n whole numbers need a different formula?
Again, an easy stupid question but its the last question I have on this chapter after really struggling with sequences and series so thanks <3
I know this is an easy question but once you've overanalysed something its hard to go back !
The sum of the first n numbers is sn = n/2 (2a+(n-1)d) or sn= n/2 (a+l)
And the sum of the first n whole numbers is sn= n/2 n(n+1)

My question is, what differs between these two formulas? When do you use each one? Why does the sum of n whole numbers need a different formula?
Again, an easy stupid question but its the last question I have on this chapter after really struggling with sequences and series so thanks <3
sn= n/2 n(n+1) cannot be right... try putting 4 for n... the answer should be 10...
I know this is an easy question but once you've overanalysed something its hard to go back !
The sum of the first n numbers is sn = n/2 (2a+(n-1)d) or sn= n/2 (a+l)
And the sum of the first n whole numbers is sn= n/2 n(n+1)

My question is, what differs between these two formulas? When do you use each one? Why does the sum of n whole numbers need a different formula?
Again, an easy stupid question but its the last question I have on this chapter after really struggling with sequences and series so thanks <3
and are for arithmetic sequences with common difference .

Whereas, is for the sequence i.e. an arithmetic sequence with first term and common difference .

So if you set and into the first formula, you will get . Likewise, and in the second formula will yield the same result.
4. The formula for the sum of the first n natural numbers is a special case of the general sum of an arithmetic series where you just add consecutive natural numbers up to n.

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: January 9, 2017
Today on TSR

### Edexcel C2 Core Unofficial Markscheme!

Find out how you've done here

Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams