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# A certain sequence is defined by the formula an = an – 1 – 7

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GMAT Club Legend
Joined: 07 Jun 2014
Posts: 4750
GRE 1: Q167 V156
WE: Business Development (Energy and Utilities)
Followers: 93

Kudos [?]: 1664 [0], given: 396

A certain sequence is defined by the formula an = an – 1 – 7 [#permalink]  30 Jul 2018, 10:28
Expert's post
00:00

Question Stats:

92% (00:45) correct 7% (03:29) wrong based on 14 sessions
A certain sequence is defined by the formula $$a_n = a_{n – 1} – 7$$.

$$a_7 = 7$$

 Quantity A Quantity B $$a_1$$ $$-35$$

A) Quantity A is greater.
B) Quantity B is greater.
C) The two quantities are equal.
D) The relationship cannot be determined from the information given.
_________________

Sandy
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GMAT Club Legend
Joined: 07 Jun 2014
Posts: 4750
GRE 1: Q167 V156
WE: Business Development (Energy and Utilities)
Followers: 93

Kudos [?]: 1664 [0], given: 396

Re: A certain sequence is defined by the formula an = an – 1 – 7 [#permalink]  12 Aug 2018, 05:41
Expert's post
Explanation

The sequence $$a_n = a_{n - 1} - 7$$ can be read as “to get any term in sequence a, subtract 7 from the previous term.” The problem provides the 7th term; plug the term into the function in order to determine the pattern.

Note that Quantity A asks for the value of a1, so try to find the 6th term:

$$7 = a_6 - 7$$

$$a_6 = 14$$

In other words, each previous term will be 7 greater than the subsequent term. Therefore, $$a_7 = 7$$, $$a_6 = 14$$, $$a_5 = 21$$, and so on.

The term $$a_1$$, then, is greater than the starting point, 7, and must also be greater than the negative value in Quantity B. Quantity A is greater. Note that the value in Quantity B is the result of incorrectly subtracting 7 six times, rather than adding it.
_________________

Sandy
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Manager
Joined: 23 Oct 2018
Posts: 58
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Kudos [?]: 0 [0], given: 0

Re: A certain sequence is defined by the formula an = an – 1 – 7 [#permalink]  25 Oct 2018, 04:37
I didnt get it
Intern
Joined: 22 Jul 2018
Posts: 41
Followers: 0

Kudos [?]: 9 [1] , given: 5

Re: A certain sequence is defined by the formula an = an – 1 – 7 [#permalink]  31 Oct 2018, 00:51
1
KUDOS
a7 = a6 - 7 = a5 - 17 = a4 -21 = a3 - 28 = a2 - 35 = a1 - 42
a7 = a1 -42
7 = a1 -42
a1 = 49
A
Re: A certain sequence is defined by the formula an = an – 1 – 7   [#permalink] 31 Oct 2018, 00:51
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# A certain sequence is defined by the formula an = an – 1 – 7

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