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# Let a1, a2, a3....a(n) be a sequence of positive number

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VP
Joined: 30 Jun 2008
Posts: 1007
Let a1, a2, a3....a(n) be a sequence of positive number [#permalink]

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30 Oct 2008, 09:17
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Let a1, a2, a3....a(n) be a sequence of positive number where a1=1 and for n>=1, a(n+1)= a(n)+4. Which of the following is the nth term in the sequence?
a)4n-3
b)4n-1
c)4n
d)4n+1
e)4n+4

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Manager
Joined: 20 Mar 2008
Posts: 147
Location: USA
Re: PS : A Sequence Problem [#permalink]

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30 Oct 2008, 09:45
It must be a)4n-3

a(1)=a(0)+4 -> a(0)=-3

a(1)= -3+ 4 * 1 =1
a(2)= -3+ 4 * 2 =5
a(3)= -3+ 4 * 3 =9
a(n)= -3+ 4 * n = 4n-3
VP
Joined: 30 Jun 2008
Posts: 1007
Re: PS : A Sequence Problem [#permalink]

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30 Oct 2008, 09:52
vishy007 wrote:
It must be a)4n-3

a(1)=a(0)+4 -> a(0)=-3

a(1)= -3+ 4 * 1 =1
a(2)= -3+ 4 * 2 =5
a(3)= -3+ 4 * 3 =9
a(n)= -3+ 4 * n = 4n-3

You are right ... can you explain how you formed the equations ??
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"You have to find it. No one else can find it for you." - Bjorn Borg

Manager
Joined: 20 Mar 2008
Posts: 147
Location: USA
Re: PS : A Sequence Problem [#permalink]

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30 Oct 2008, 09:57
QS says for n>=1, a(n+1)= a(n)+4

Only value we know is a(1), so I checked whether we could derive some info by using only available value.

Thus I arrived at a(1)= a (0+1)= a(0)+4 . Equating this with 1 I got a(0)=-3
VP
Joined: 30 Jun 2008
Posts: 1007
Re: PS : A Sequence Problem [#permalink]

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30 Oct 2008, 10:03
vishy007 wrote:
QS says for n>=1, a(n+1)= a(n)+4

Only value we know is a(1), so I checked whether we could derive some info by using only available value.

Thus I arrived at a(1)= a (0+1)= a(0)+4 . Equating this with 1 I got a(0)=-3

Thanks vishy ... I am not getting this part
Quote:
a(1)= -3+ 4 * 1 =1
a(2)= -3+ 4 * 2 =5
a(3)= -3+ 4 * 3 =9
a(n)= -3+ 4 * n = 4n-3

_________________

"You have to find it. No one else can find it for you." - Bjorn Borg

Manager
Joined: 20 Mar 2008
Posts: 147
Location: USA
Re: PS : A Sequence Problem [#permalink]

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30 Oct 2008, 10:08
a(2)= -3+ 4 * 2 =5
a(1)= a(0)+ 4 = -3 + 4 * 1
a(2)= a(1) + 4 = a(0) + 4 + 4 = a(0) + 4 * 2 = -3 + 4 * 2
....

Hope this helps.
VP
Joined: 30 Jun 2008
Posts: 1007
Re: PS : A Sequence Problem [#permalink]

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30 Oct 2008, 10:16
vishy007 wrote:
a(2)= -3+ 4 * 2 =5
a(1)= a(0)+ 4 = -3 + 4 * 1
a(2)= a(1) + 4 = a(0) + 4 + 4 = a(0) + 4 * 2 = -3 + 4 * 2
....

Hope this helps.

Ahh !! Get it now Thanks ......
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Current Student
Joined: 28 Dec 2004
Posts: 3288
Location: New York City
Schools: Wharton'11 HBS'12
Re: PS : A Sequence Problem [#permalink]

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30 Oct 2008, 10:29
its simple just plug numbers

n=1 a(2)=5
n=2 a(3)=9
n=3 (a4)=13
n=4 a(5)=17

so when n=4 the series is 17

4n+1

i disagree that a is the answer..

key word in the stem sayd for n>=1 if it had just said n>1 then i agree..
VP
Joined: 30 Jun 2008
Posts: 1007
Re: PS : A Sequence Problem [#permalink]

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30 Oct 2008, 10:35
amitdgr wrote:
Let a1, a2, a3....a(n) be a sequence of positive number where a1=1 and for n>=1, a(n+1)= a(n)+4. Which of the following is the nth term in the sequence?
a)4n-3
b)4n-1
c)4n
d)4n+1
e)4n+4

I found another approach ...

a(n+1)= a(n)+4 implies a(n+1) - a(n) = 4

this means d=4

nth term a(n) = a + (n-1)d = 1 + (n-1)*4 = 1 + 4n - 4 = 4n-3
_________________

"You have to find it. No one else can find it for you." - Bjorn Borg

Current Student
Joined: 28 Dec 2004
Posts: 3288
Location: New York City
Schools: Wharton'11 HBS'12
Re: PS : A Sequence Problem [#permalink]

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30 Oct 2008, 11:45
i should add, the problem with such type questions is English..i feel like i get confused what they are asking about?

here the question is asking what is a(n)...had i known that it was obvious the ans choice would be 4n-3...

however, what got me was that i was looking for the nth term blindly..not realizing what the question is asking..

if n=4..then the a(5)=4n+1...

so i think it is a good thing to take a few moments just to absorb what the question is asking...

so i guess it will help if you spend the first 30 sec trying to just understand what is being asked..screw the time limit..i think if you are like me the math is mechanical once you have figured what you need to get..

so i guess the biggest challenge for most of us is understanding what is being asked..

i would welcome any tips on how to expiditiously understand what is being asked..
SVP
Joined: 29 Aug 2007
Posts: 2426
Re: PS : A Sequence Problem [#permalink]

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30 Oct 2008, 13:12
vishy007 wrote:
It must be a)4n-3

a(1)=a(0)+4 -> a(0)=-3

a(1)= -3+ 4 * 1 =1
a(2)= -3+ 4 * 2 =5
a(3)= -3+ 4 * 3 =9
a(n)= -3+ 4 * n = 4n-3

:cool

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If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.

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Re: PS : A Sequence Problem   [#permalink] 30 Oct 2008, 13:12
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