You are Here: Home >< Maths

sequences help Watch

1. can someone pls explain part d???? what i did was find the sum of the first 3 (1+(-1/2) +1) then multiplied it by 100/3 and i got an answer of 50 . why is this wrong?

plus this is not arithmetic so how did it use that rule
Attached Images

2. Write out the first 4 terms, notice anything?
The sequence repeats every 4 terms, 1,-1/2,1,-1/2 ...
If we are summing the first 100 terms then how many lots of this sequence is there?
3. (Original post by pondsteps)
can someone pls explain part d???? what i did was find the sum of the first 3 (1+(-1/2) +1) then multiplied it by 100/3 and i got an answer of 50 . why is this wrong?

plus this is not arithmetic so how did it use that rule
the first 3 terms as they are in the sequence don't repeat i.e in this sequence the first 3 terms goes 1, -0.5, 1

when you add the first 3 terms multiply by 100 then divide by 3 you are assuming the sequence goes like this

[1,-0.5,1,][1,-0.5,1] etc etc
because you've assumed the firts 3 terms repeat one after another, but they don't

an arithmetic sequence is one where the difference between the terms is constant
4. (Original post by pondsteps)
can someone pls explain part d???? what i did was find the sum of the first 3 (1+(-1/2) +1) then multiplied it by 100/3 and i got an answer of 50 . why is this wrong?

plus this is not arithmetic so how did it use that rule
There should either be a general pattern in terms or it may be an arithmetic

What is ?
5. (Original post by EqualRights)
There should either be a general pattern in terms or it may be an arithmetic

What is ?
fist term second term and third term
6. (Original post by thefatone)
the first 3 terms as they are in the sequence don't repeat i.e in this sequence the first 3 terms goes 1, -0.5, 1

when you add the first 3 terms multiply by 100 then divide by 3 you are assuming the sequence goes like this

[1,-0.5,1,][1,-0.5,1] etc etc
because you've assumed the firts 3 terms repeat one after another, but they don't

an arithmetic sequence is one where the difference between the terms is constant
ohhh... thank u sooo much!! xxx
7. (Original post by thefatone)
fist term second term and third term
no omg, I meant as in values haha
8. (Original post by EqualRights)
There should either be a general pattern in terms or it may be an arithmetic

What is ?
okay so i can use the rule 1/2 (n) (a+L) even if the sequence is not arithmetic??
9. (Original post by pondsteps)
okay so i can use the rule 1/2 (n) (a+L) even if the sequence is not arithmetic??
No. That only works for arithmetic sequences.
10. (Original post by B_9710)
No. That only works for arithmetic sequences.
umm but the above example is not arithmetic ... and i thought it was only for arithmetic
11. (Original post by pondsteps)
umm but the above example is not arithmetic ... and i thought it was only for arithmetic
Exactly, so you can't use that formula.
12. (Original post by Zacken)
Exactly, so you can't use that formula.
lol but the markscheme said accept and it worked 😁
13. The first 2 terms sum to 1/2 and there are 50 of them so the full sum is 25.
14. (Original post by pondsteps)
lol but the markscheme said accept and it worked 😁
Yes, the original sequence isn't arithmetic but you can sum every two terms and treat it as an arithmetic (common difference 0) sequence with 50 terms.
15. (Original post by EqualRights)
no omg, I meant as in values haha
well first term is given as 1 then use the reccurence relation to fin the next 2 ^-^

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: April 8, 2016
Today on TSR

Should I ask for his number?

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

Chat with other maths applicants

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