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C1 Arithmetic Sequence question
Hey!
I'm having a bit of trouble with this question. Wondering if anyone can help me out on part C?
The first term of an arithmetic sequence is 30 and the common difference is -1.5.
a) Find the value of the 25th term.
I get this U25 = -6 which is correct.
b) The rth term of the sequence is 0. Find the value of r.
I found r = 21.
c) The sum of the first n terms of the sequence is Sn. Find the largest possible value of Sn.
I have no clue how to go about part c. I'm stuck!
Thank you.
I'm having a bit of trouble with this question. Wondering if anyone can help me out on part C?
The first term of an arithmetic sequence is 30 and the common difference is -1.5.
a) Find the value of the 25th term.
I get this U25 = -6 which is correct.
b) The rth term of the sequence is 0. Find the value of r.
I found r = 21.
c) The sum of the first n terms of the sequence is Sn. Find the largest possible value of Sn.
I have no clue how to go about part c. I'm stuck!
Thank you.
Reply 1
the common difference is -1.5 so each term is 1.5 smaller than the last. so at some point it will start going into negative numbers
Reply 3
I might be totally wrong here..
I think the key is in part B because if you go any further you're going to be adding minus numbers which will decrease your total so...
I think you use the formula in the formula book 0.5n(a+l) when n=21
that being.. 0.5(21)(30+0)=315
correct me please if i'm wrong
edit: done in the time I took to type it
I think the key is in part B because if you go any further you're going to be adding minus numbers which will decrease your total so...
I think you use the formula in the formula book 0.5n(a+l) when n=21
that being.. 0.5(21)(30+0)=315
correct me please if i'm wrong
edit: done in the time I took to type it
seanweasel
I might be totally wrong here..
I think the key is in part B because if you go any further you're going to be adding minus numbers which will decrease your total so...
I think you use the formula in the formula book 0.5n(a+l) when n=21
that being.. 0.5(21)(30+0)=315
correct me please if i'm wrong
edit: done in the time I took to type it
I think the key is in part B because if you go any further you're going to be adding minus numbers which will decrease your total so...
I think you use the formula in the formula book 0.5n(a+l) when n=21
that being.. 0.5(21)(30+0)=315
correct me please if i'm wrong
edit: done in the time I took to type it
Can you tell me what you got for the answer please?
Reply 6
I got 885 for the sum and n is 20 not 21,they're asking for the last POSITIVE value,which is 1.5.
and n is 20 not 21,they're asking for the last POSITIVE value,which is 1.5.
Original post by SyrupandHoney
I got 885 for the sum and n is 20 not 21,they're asking for the last POSITIVE value,which is 1.5.
and n is 20 not 21,they're asking for the last POSITIVE value,which is 1.5.You need to check your numbers again.
It does not matter for n as one of the terms will be 0, so the sum stays. But in any case, the sum won't be 885.
Reply 11
Original post by casperyc
This should help you to check some answers and make sense of what's happening. The graph is plotted for n against Sn.
Good that you want to help OP but the post is from 9 years ago...
Original post by BobbJo
Good that you want to help OP but the post is from 9 years ago...
OMG........
Didn't even notice. I was not the first one to reply though @Riamilton
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