You are Here: Home >< Maths

# C1 Sigma Notation help Watch

1. okay basically this is the question:

its the sigma sight with 5 on the top, r= 0 on the bottom and the equation things is r(r+1).

I keep getting 70 but the answers at the back of the book say its 112.

zeroth term, 1st term, 2nd term, 3rd term, fourth term, fifth term.
0 + 2 + 6 + 12 + 20 + 30

I didn't use sn = 0.5n (2a +(n-1)d) becuase the difference keeps on changing (so there is no common difference ? not sure).

So yeahh, what am i doing wrong ??
2. Well if the question is exactly as you say then yes 70 is the right answer, 112 comes from doing the sum from r=1 to 6 so either youve read the question wrong or the book is wrong.
3. (Original post by applestrudle)

I didn't use sn = 0.5n (2a +(n-1)d) becuase the difference keeps on changing (so there is no common difference ? not sure).
Yes, expand out r(r+1) and this makes r^2 + r, so the difference between them is getting bigger by 2 each time as it is r^2.
4. From looking at my stats I just found the formula which can be used for when there is a ^2 question:

Sn = n/6 (n+1)(2n+1)

So with 5 terms, n=5 :

5/6 * 6 * 11 = 55

But it's r^2 +r, so add on 5+4+3+2+1 = 15

So 55 + 15 = 70

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: November 28, 2010
Today on TSR

### Degrees to get rich!

... and the ones that won't

### Women equal with Men?

Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• 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

## Groups associated with this forum:

View associated groups
Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.