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# sequences help

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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
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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 ^-^

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