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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GMAT Prep - Arithmetic Sequence
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sangeethai
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camitava
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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.
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
Regards,
Amitava
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siddarthd2919
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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
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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camitava
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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.
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
Regards,
Amitava
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sangeethai
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Hi Amitava and Siddharth,
Thanks for your help. I have learned to solve this problem.
regards,
Sangeetha
Thanks for your help. I have learned to solve this problem.
regards,
Sangeetha
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camitava
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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
Regards,
Amitava
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.
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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edvhou812
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Keep it simple, stupid. I'd thank this post 10 times if I could.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.
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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studentps2011
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Wow! that's a cool way to look at this problemdisccomp 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.
