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GMAT Prep - Arithmetic Sequence

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by sangeethai » Thu Feb 21, 2008 4:47 pm
In the arithmetic sequence t1,t2,t3,......tn......, t1 = 23 and tn = tn-1 - 3 for each n>1. What is the value of n when tn = -4

1) -1
2) 7
3) 10
4) 14
5) 20

Correct ans is 3) 10

Please shed some light on this problem. Thanks.
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by camitava » Thu Feb 21, 2008 6:12 pm
sangeethai,
u can take tn = -4 as the first term of the series. now use the formula -
tn = a + (n - 1) * d
where tn = -4, a = -4 and d = 3 , u can get n = 10.
Correct me If I am wrong


Regards,

Amitava
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by siddarthd2919 » Thu Feb 21, 2008 7:19 pm
hi camitava,

the formula u gave is wrong. the formula is

an=a1+(n-1)*d

we have an=23,a1=-4

23 = -4 +(n-1)*3
23 = -4 +3n-3
3n = 30

n= 10
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by camitava » Fri Feb 22, 2008 4:33 am
siddarthd2919,
if u check my post again, u will find that i have taken -4 as the first term and 3 as the common diff. So this comes same if u take 23 as first term, -4 as nth term and -3 as common diff.
Te main thing is the formula - tn = a + (n - 1) * d - now the way u will use this formula depends on u.
Correct me If I am wrong


Regards,

Amitava
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Thanks - Knowing the formula really help

by sangeethai » Fri Feb 22, 2008 6:58 am
Hi Amitava and Siddharth,
Thanks for your help. I have learned to solve this problem.

regards,
Sangeetha
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by khurram » Fri Feb 22, 2008 10:23 am
hello

What heading would this sort of formula be found under. Did not know such a formula existed so want to revise it.

Also, how did you guys calculate d as 3.
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by camitava » Fri Feb 22, 2008 10:28 am
khurram, d is he common difference. as te question says tn = tn-1 - 3. so the common diff is 3 or -3 - the way u will take. d or common diff means the diff between two consecutive terms of the series (AP - Arithmetic Progression). u will get more input f u do some google search on arithmetic progression.
Correct me If I am wrong


Regards,

Amitava
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by khurram » Fri Feb 22, 2008 12:00 pm
Thanks will do tonight

khurram
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by disccomp » Mon Aug 08, 2011 12:35 pm
I was trying to wrap my mind around the sequential number formula, When I realized the problem is a simple linear function, just use the slope intercept formula:
y = mx + b

x => n
y => an
m(slope) => an-1 (The difference between two consecutive numbers in a sequence)
b(y-intercept) =>c

So for the given problem:
23 = -3(1) + b, solve for b: b=26

-4 = -3n + 26, subtract 26 from both sides
-30 = -3n, solve for n: n = 10

No need to memorize a separate formula.
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by edvhou812 » Wed Sep 28, 2011 10:41 pm
disccomp wrote:I was trying to wrap my mind around the sequential number formula, When I realized the problem is a simple linear function, just use the slope intercept formula:
y = mx + b

x => n
y => an
m(slope) => an-1 (The difference between two consecutive numbers in a sequence)
b(y-intercept) =>c

So for the given problem:
23 = -3(1) + b, solve for b: b=26

-4 = -3n + 26, subtract 26 from both sides
-30 = -3n, solve for n: n = 10

No need to memorize a separate formula.
Keep it simple, stupid. I'd thank this post 10 times if I could.
I don't know what to say, really. Three minutes to the biggest battle of our professional lives. You find out life's this game of inches, so is football. Because in either game - life or football - the margin for error is so small. I mean, one half a step too late or too early and you don't quite make it. One half second too slow, too fast and you don't quite catch it. I'll tell you this, in any fight it's the guy whose willing to die whose gonna win that inch. That's football guys, that's all it is. Now, what are you gonna do?
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by studentps2011 » Thu Sep 29, 2011 9:55 am
disccomp wrote:I was trying to wrap my mind around the sequential number formula, When I realized the problem is a simple linear function, just use the slope intercept formula:
y = mx + b

x => n
y => an
m(slope) => an-1 (The difference between two consecutive numbers in a sequence)
b(y-intercept) =>c

So for the given problem:
23 = -3(1) + b, solve for b: b=26

-4 = -3n + 26, subtract 26 from both sides
-30 = -3n, solve for n: n = 10

No need to memorize a separate formula.
Wow! that's a cool way to look at this problem
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