# arithmetic series helpWatch

#1
The sum of the first three numbers of an arithmetic series is 12. If the 20th term is -32, find the first term and the common difference.

how do i do this?
0

4 years ago
#2
Use the formula for the sum of an arithmetic series for the 3 first values. Use the formula for Un=U20 = -32. Solve simultaneously.
0
#3
(Original post by TVIO)
Use the formula for the sum of an arithmetic series for the 3 first values. Use the formula for Un=U20 = -32. Solve simultaneously.
I did the formula and got 3a+3d=12. So what do I do now?
0
#4
Nevermind, I've worked it out. Thanks to TVIO.
0
4 years ago
#5
(Original post by Year11guy)
I did the formula and got 3a+3d=12. So what do I do now?
You know that the 20th number is -32, so use the formula for Un and use it for U(20) = -32
0
#6
(Original post by TVIO)
You know that the 20th number is -32, so use the formula for Un and use it for U(20) = -32
Thanks for the help, I've worked it out now!
0
4 years ago
#7
(Original post by Year11guy)
Thanks for the help, I've worked it out now!
Glad to help! I personally found some of those sequences questions the hardest things in the course as you have to be really careful with them.
0
X

new posts
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

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

### University open days

• Cranfield University
Cranfield Forensic MSc Programme Open Day Postgraduate
Thu, 25 Apr '19
• University of the Arts London
Open day: MA Footwear and MA Fashion Artefact Postgraduate
Thu, 25 Apr '19
• Cardiff Metropolitan University
Sat, 27 Apr '19

### Poll

Join the discussion

#### Have you registered to vote?

Yes! (236)
39.14%
No - but I will (41)
6.8%
No - I don't want to (45)
7.46%
No - I can't vote (<18, not in UK, etc) (281)
46.6%