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# In the arithmetic sequence t1, t2, t3, ..., tn, t1=23 and tn

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In the arithmetic sequence t1, t2, t3, ..., tn, t1=23 and tn  [#permalink]

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20 Mar 2012, 08:00
5
47
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15% (low)

Question Stats:

81% (01:46) correct 19% (01:54) wrong based on 524 sessions

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In the arithmetic sequence $$t_1$$, $$t_2$$, $$t_3$$, ..., $$t_n$$, $$t_1=23$$ and $$t_n= t_{n-1} - 3$$ for each n > 1. What is the value of n when $$t_n = -4$$?

A. -1
B. 7
C. 10
D. 14
E. 20

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Re: In the arithmetic sequence t1, t2, t3, ..., tn, t1=23 and tn  [#permalink]

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20 Mar 2012, 12:44
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enigma123 wrote:
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?

A. -1
B. 7
C. 10
D. 14
E. 20

$$t_n=t_{n-1}-3$$ means that each term is 3 less than the previous term. Now, the difference between $$t_1=23$$ and $$t_n=-4$$ is $$23-(-4)=27$$, so we moved $$\frac{27}{3}=9$$ terms from $$t_1$$, so from $$t_1$$ to $$t_{10}$$.

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Re: In the arithmetic sequence t1, t2, t3, ..., tn, t1=23 and tn  [#permalink]

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12 Apr 2012, 09:24
16
3
tn= tn-1 - 3 means that d=-3

tn=t1 +d(n-1)
-4=23-3(n-1)
-30=-3n
n=10
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Re: In the arithmetic sequence t1, t2, t3, ..., tn, t1=23 and tn  [#permalink]

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20 Mar 2012, 08:42
10
5
well.....t2 =t1-3 =23-3=20

t3=t2-3=20-3=17

So every time we n increases tn decreases by 3.

Since t1=23 we have 23-3=20-3=17-3=14-3=11-3=8-3=5-3=2-3=-1-3=-4!VOILA

So n=10 ten times substructing 3 from 23 to reach -4.

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Re: In the arithmetic sequence t1, t2, t3, ..., tn, t1=23 and tn  [#permalink]

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22 Apr 2014, 01:15
7
2
23... 20.... 17... 14... 11.... 8... 5... 2.... -1.... -4

-4 is the 10th term

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Re: In the arithmetic sequence t1, t2, t3, ..., tn, t1=23 and tn  [#permalink]

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03 Feb 2015, 00:44
2
I also did the same as Paresh, but like this:

We know that t1 = 23
So, using the given formula we have:
t1=(t1-1) -3 =23
t0 - 3 = 23
t0= 26

The sam way we find that t2= 20

It seems that the sequence goes like this:
t0 = 26
t1 = 23
t2 = 20
t3 = 17
t4 = 14
t5 = 11
t6 = 8
t7 = 5
t8 = 2
t9 = -1
t10 = -4

So, our ANS is C.

However, I did do it wrong because, by the way I saw it writen, I though that the whole formula equaled to 23 (t1, t2, t3, ..., tn, t1=23). I didn't see any relationship as to why this is 23 (it is an addition, is it a multiplication?). So, then I thought that what was meant is that this sequence is doing a circle, going from t1 to t1 again, and there are 23 numbers in the sequence. So, I though we needed to find tn.

Hopefully the gmat would show in a more clear way that t1=23 and not the whole sequence..
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Re: In the arithmetic sequence t1, t2, t3, ..., tn, t1=23 and tn  [#permalink]

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08 Jan 2017, 23:59
2
t1=23
t2=t1-3=20
t3=t2-3=17 and so on...
Here is when we need to consider the formula for AP as we know the common difference is -3

tn=t1 + d(n-1)

given, tn=-4
-4=23 + (-3) (n-10) >> n=10

Ans : C
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Re: In the arithmetic sequence t1, t2, t3, ..., tn, t1=23 and tn  [#permalink]

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09 Jan 2017, 02:17
1
enigma123 wrote:
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?

A. -1
B. 7
C. 10
D. 14
E. 20

Its a normal arithmetic Progression Question , whose 1st term is 23 and common difference is -3

Tn = a+ (n-1)d

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

n = 10 .
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Re: In the arithmetic sequence t1, t2, t3, ..., tn, t1=23 and tn  [#permalink]

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11 Jan 2017, 07:09
1
1
1
enigma123 wrote:
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?

A. -1
B. 7
C. 10
D. 14
E. 20

In the given sequence, since we are given the first term, we can use that value to find the second term, and once we know the second term, we can use that value to find the third term, and so on.

t_1 = 23

t_2 = t_1 – 3 = 23 – 3 = 20

t_3 = t_2 – 3 = 20 – 3 = 17

t_4 = t_3 – 3 = 17 – 3 = 14

t_5 = t_4 – 3 = 14 – 3 = 11

t_6 = t_5 – 3 = 11 – 3 = 8

t_7 = t_6 – 3 = 8 – 3 = 5

t_8 = t_7 – 3 = 5 – 3 = 2

t_9 = t_8 – 3 = 2 – 3 = -1

t_10 = t_9 – 3 = -1 – 3 = -4

So n = 10.

Alternative Solution:

Notice that starting from the second term, each term is 3 less than the previous term, which makes the sequence an arithmetic sequence. In an arithmetic sequence, the nth term, a_n, can be found by using the formula a_n = a_1 + d(n – 1) in which a_1 is the first term and d is the common difference.

Since we are given t_n, we can modify the formula to t_n = t_1 + d(n – 1) in which t_1 = 23 and d = -3. So we have:

t_n = t_1 + d(n – 1)

-4 = 23 + (-3)(n – 1)

-27 = -3(n – 1)

9 = n – 1

10 = n

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Re: In the arithmetic sequence t1, t2, t3, ..., tn, t1=23 and tn  [#permalink]

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08 Jun 2017, 15:44
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Attached is a visual that should help.
Attachments

Screen Shot 2017-06-08 at 4.33.09 PM.png [ 108.8 KiB | Viewed 60677 times ]

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Re: In the arithmetic sequence t1, t2, t3, ..., tn, t1=23 and tn  [#permalink]

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05 Jul 2017, 18:18
Here is how I did it:

t1 = 23 and tn-1 = -3; therefore for every t you need to subtract 3

lets begin:

t1----> 23-3 = 20
t2----> 20-3 = 17
t3----> 17-3 = 14

when tn = 4...now subtract 4 from the 14

tn----> 14-4 = 10

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Re: In the arithmetic sequence t1, t2, t3, ..., tn, t1=23 and tn  [#permalink]

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28 Feb 2018, 21:41
Hi All,

Questions that use "sequence notation" are relatively rare on Test Day (you'll probably see just 1), but the math behind the sequence is usually some fairly simple arithmetic (add, subtract, multiply, divide).

Here, we're given the first term in the sequence (23) and we're told that each term thereafter is 3 LESS than the preceding term. Once you understand how the sequence "works", in many cases, it's really easy to just "map out" the sequence. We're asked which term in the sequence equals -4.....

1st = 23
2nd = 20
3rd = 17
4th = 14
5th = 11
6th = 8
7th = 5
8th = 2
9th = -1
10th = -4

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Special Offer: Save $75 + GMAT Club Tests Free Official GMAT Exam Packs + 70 Pt. Improvement Guarantee www.empowergmat.com/ *****Select EMPOWERgmat Courses now include ALL 6 Official GMAC CATs!***** Intern Joined: 07 May 2018 Posts: 2 Re: In the arithmetic sequence t1, t2, t3, ..., tn, t1=23 and tn [#permalink] ### Show Tags 07 May 2018, 11:41 a sequence of numbers contains terms t1, t2, t3,......... , tn, where every term is equal to the sum of its two preceding terms. t5 =18 and t8 = 76 what is the value of t9 Answer this EMPOWERgmat Instructor Status: GMAT Assassin/Co-Founder Affiliations: EMPOWERgmat Joined: 19 Dec 2014 Posts: 13574 Location: United States (CA) GMAT 1: 800 Q51 V49 GRE 1: Q170 V170 Re: In the arithmetic sequence t1, t2, t3, ..., tn, t1=23 and tn [#permalink] ### Show Tags 07 May 2018, 15:12 Hi Mangalshubham, In the future, when posting questions, you should post each in its own unique thread AND you should make sure to post the ENTIRE prompt (with the 5 answer choices and the correct answer - hidden behind a "spoiler" tag). The prompt you've submitted is a sequence question that you can use to create the following equations. We're told that each term is equal to the SUM of the TWO terms that immediately precede it... 5th term = 18 6th term = X 7th term = 18+X 8th term = 76 = (X + 18 + X) 76 = 2X + 18 58 = 2X 29 = X Thus, we have... 5th term = 18 6th term = 29 7th term = 47 8th term = 76 9th term = ? Final Answer: 47+76 = 123 GMAT assassins aren't born, they're made, Rich _________________ 760+: Learn What GMAT Assassins Do to Score at the Highest Levels Contact Rich at: Rich.C@empowergmat.com # Rich Cohen Co-Founder & GMAT Assassin Special Offer: Save$75 + GMAT Club Tests Free
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Re: In the arithmetic sequence t1, t2, t3, ..., tn, t1=23 and tn  [#permalink]

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07 May 2018, 20:10
Empower GmatRich

Thank you for solution... and this question doen not have answer choices.. it is weather the 9th term is equal or less than 126

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In the arithmetic sequence t1, t2, t3, ..., tn, t1=23 and tn  [#permalink]

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20 Feb 2019, 07:53
Bunuel wrote:
enigma123 wrote:
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?

A. -1
B. 7
C. 10
D. 14
E. 20

$$t_n=t_{n-1}-3$$ means that each term is 3 less than the previous term. Now, the difference between $$t_1=23$$ and $$t_n=-4$$ is $$23-(-4)=27$$, so we moved $$\frac{27}{3}=9$$ terms from $$t_1$$, so from $$t_1$$ to $$t_{10}$$.

I have always trouble to understand what the question asks, once understood it is easy. I thought we should search what the result of t(-4) is so I thought t1 is 23 that means t(-1) has to be 3 more, because its movie backwards and t(-2) is therefore 29...t/-3( 32, t(-4)=35!, not that we should search for which tn has the result-4.

How can I fix my problem ?
In the arithmetic sequence t1, t2, t3, ..., tn, t1=23 and tn   [#permalink] 20 Feb 2019, 07:53
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