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# Sequences and series watch

1. Find the sum of:

41 + 38 + 35 + ... + (-19)

I thought I would use the formula 0.5n(a+l) but I don't get the same answer which the book gives. I must be doing something wrong- help please! The answer is 231.

and then... the first term is -11 and the ninth term of the series is 1
Find the common diffrence and teh sum of the first twenty terms of teh series.

Any explanation would be appreciated. I've managed to answer all of the questions without negative numbers- so it's got to be there that I'm going wrong. Thanks
2. (Original post by Elm_lily)
Find the sum of:

41 + 38 + 35 + ... + (-19)

I thought I would use the formula 0.5n(a+l) but I don't get the same answer which the book gives. I must be doing something wrong- help please! The answer is 231.

and then... the first term is -11 and the ninth term of the series is 1
Find the common diffrence and teh sum of the first twenty terms of teh series.

Any explanation would be appreciated. I've managed to answer all of the questions without negative numbers- so it's got to be there that I'm going wrong. Thanks

I expect you got the number of terms wrong. Consider 41, 38, 35,32
Difference = 9
9 divided by 3 =3
Number of terms = 3+1=4
3. It's an arithmetic series, not a geometric...
4. (Original post by maxfire)
It's an arithmetic series, not a geometric...
Neither the OP or I have suggested it is geometric. I don't see why you think otherwise.
5. (Original post by steve2005)
Neither the OP or I have suggested it is geometric. I don't see why you think otherwise.
Didn't read the question properly did I?
Spoiler:
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