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by gmatrant » Tue Nov 20, 2007 6:36 pm
Its a AP series.

Term = a + (n-1)d
where a = first term
n = nth term
d = difference

So here a =23, d=-3 and we need to find the nth term whose value is -4.

23+ (n-1)*-3 = -4
-3n+3 = -27
n = 10

Is the OA 10
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by xcise_science » Tue Nov 20, 2007 7:37 pm
Thanks, but I don't understand what you meant here:

Term = a + (n-1)d
where a = first term
n = nth term
d = difference

So here a =23, d=-3 and we need to find the nth term whose value is -4.
How did you know the first term was 23?

Also, whats an AP? Arithmetic progression?

Thanks
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by ri2007 » Wed Nov 21, 2007 6:12 am
gmatrant

I got the same sanswer as u, used the same AP formula :)) but when I saw the ans given by the GMAT prep people it is 14.

Any thoughts?
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by jrbrown2 » Wed Nov 21, 2007 8:45 am
I used the AP formula and did it manually
tn=t1 + (n-1)d
-4=23+(n-1)(-3)
n=10

23, 20, 17, 14, 11, 8, 5, 2, -1, -4 <----10th term

can't see how 14 is the answer (t1 would have to be 35 for n to be 14).
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by xcise_science » Wed Nov 21, 2007 9:00 am
But I'm still somewhat unclear as to this AP thing, especially this: tn=t1 + (n-1)d
where do I get more information on how to attack this?

thanks!
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by jrbrown2 » Wed Nov 21, 2007 9:14 am
It's an arithmetic sequence. the question states that each consecutive term is 3 less than the previous.

for instance, the 3rd term would be 3 less than the second term, which is 3 less than the first term (hope thats clear)

1st term = 23
2nd term = 23-3 or 23+(1)(-3) = 20
3rd term = 23-3-3 or 23+(2)(-3) = 17
4th term = 23-3-3-3 or 23+(3)(-3) = 14
and so on...

To rewrite this into a formula, the value of the nth term would be 23+(n-1)(-3)
i.e. Value of nth term = 1st term + (n -1)(difference)
or
Tn = T1 + (n-1)d
Last edited by jrbrown2 on Wed Nov 21, 2007 10:34 am, edited 4 times in total.
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by ri2007 » Wed Nov 21, 2007 9:14 am
https://www.mathcentre.ac.uk/resources/l ... ssions.pdf

refer to this link for more details
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by xcise_science » Wed Nov 21, 2007 10:14 am
OOPS! forgive me! I didn't see that little comma that said T(1)=23!
For some reason, I thought that was a continuation from T(n)...
Now it makes sense, cos I was wondering how you determined that T(1)=23..

Thanks for your help!
(so can this formula be used for any sortof progression question?)
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by jrbrown2 » Wed Nov 21, 2007 10:32 am
this formula can only be used for addition.
For different types of progressions, check that link that r12007 posted. It's pretty good.
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by Jimat » Thu Nov 22, 2007 9:41 am
I still don`t get it. The questions says what is the value of N when Tm =-4, what jrbrown has answered is what is the value of the 4th term.
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