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TAGS: GRE Prep Club Legend  Joined: 07 Jun 2014
Posts: 4810
GRE 1: Q167 V156 WE: Business Development (Energy and Utilities)
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term after the first term is equal to the preceding term [#permalink]
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Expert's post 00:00

Question Stats: 58% (01:00) correct 41% (00:23) wrong based on 12 sessions
$$a_1, a_2, a_3 ................ a_n$$

In the sequence above, each term after the first term is equal to the preceding term plus the constant c. If $$a_1+a_3+a_5=27$$, what is the value of $$a_2+a_4$$ ?

$$a_2+a_4=$$

[Reveal] Spoiler: OA
18

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Sandy
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Try our free Online GRE Test GRE Prep Club Legend  Joined: 07 Jun 2014
Posts: 4810
GRE 1: Q167 V156 WE: Business Development (Energy and Utilities)
Followers: 117

Kudos [?]: 1895  , given: 397

Re: term after the first term is equal to the preceding term [#permalink]
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Expert's post
Solution

This is clearly an arithmetic progression and we just need the first 5 terms. We can rewrite the progression as a - 2c, a - c, a , a + c, a + 2c. Here the difference between the terms is c as described in the problem

a1 + a3 + a5 = a - 2c + a + a+ 2c = 3a = 27. Hence a =9.

To find a2 + a4 = a - c + a + C = 2a =18.

The purpose of writing this series in the above form is to eliminate c as a variable. Hence answer is 18.
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Sandy
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Re: term after the first term is equal to the preceding term [#permalink]
i don't understand where you derive a - 2c and a + 2c from Director  Joined: 16 May 2014
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GRE 1: Q165 V161 Followers: 97

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Re: term after the first term is equal to the preceding term [#permalink]
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Lets take a real arithmetic progression:

1, 5, 9 , 13, 17

As you can see any term of the above arithmetic progression is 4 greater than the previous term. 1+4 = 5... 5+4 = 9.... so on

So if we draw parallels with the question above here c = 4. Then we can write the above series taking the middle term 9 into account:
9-2*4, 9-4, 9 , 9+4, 9+2*4
a-2*c, a-c , a , a+c, a+2*c

Thats how Sandy derives the a+2c and a-2c terms
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If you find this post helpful, please press the kudos button to let me know !  Intern Joined: 26 May 2017
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Re: term after the first term is equal to the preceding term [#permalink]
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If that is confusing then, you can say a, a+c, a+2c, a+3c, a+4c

a1+a3+a5 = 27

a + a+2c + a+4c = 27
a = 9-2c

now a2 + a4

a+c + a+3c
2a+4c
2(9-2c) + 4c
18-4c+4c GRE Instructor Joined: 10 Apr 2015
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Re: term after the first term is equal to the preceding term [#permalink]
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Expert's post
sandy wrote:
$$a1, a2, a3 ................ an$$

In the sequence above, each term after the first term is equal to the preceding term plus the constant c. If $$a1+a3+a5=27$$, what is the value of $$a2+a4$$ ?

$$a2+a4=$$

[Reveal] Spoiler: OA
18

By definition:
a1 = a1
a2 = a1 + c
a3 = a1 + c + c = a1 + 2c
a4 = a1 + c + c + c = a1 + 3c
a5 = a1 + c + c + c + c = a1 + 4c

GIVEN: a1+a3+a5=27
We can write: a1 + (a1 + 2c) + (a1 + 4c) = 27
Simplify: 3a1 + 6c = 27
Divide both sides by 3 to get: a1 + 2c = 9

What is the value of $$a2+a4$$
a2 + a4 = (a1 + c) + (a1 + 3c)
= 2a1 + 4c
= 2(a1 + 2)
= 2(9)
= 18

Cheers,
Brent
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Brent Hanneson – Creator of greenlighttestprep.com  Re: term after the first term is equal to the preceding term   [#permalink] 05 Feb 2019, 12:13
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