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Math Expert V
Joined: 02 Sep 2009
Posts: 58453
If 3^a – 3^(a – 2) = 8(3^27), what is the value of 2a ?  [#permalink]

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Difficulty:   45% (medium)

Question Stats: 67% (01:42) correct 33% (01:28) wrong based on 148 sessions

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If $$3^a – 3^{(a – 2)} = 8(3^{27})$$, what is the value of 2a ?

A. 20
B. 25
C. 27
D. 29
E. 58

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Re: If 3^a – 3^(a – 2) = 8(3^27), what is the value of 2a ?  [#permalink]

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Given $$3^a – 3^{(a – 2)} = 8(3^{27})$$, we have been asked to find the value of 2a.

$$8(3^{27}) = (3^2 - 1)(3^27) = 3^{2+27} - 3{27}$$

Therefore we know that $$3^a – 3^{(a – 2)} = 3^{29} - 3{27} = 3^{29} - 3{(29 - 2)}$$

Therefore, a = 29 and 2a = 58(Option E)

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Re: If 3^a – 3^(a – 2) = 8(3^27), what is the value of 2a ?  [#permalink]

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Bunuel wrote:
If $$3^a – 3^{(a – 2)} = 8(3^{27})$$, what is the value of 2a ?

A. 20
B. 25
C. 27
D. 29
E. 58

3^a - 3^(a-2) = 8(3^27)
3^a -(3^a/3^2) = 8(3^27)
take lcm = 3^2 and then cross multiply the denominator
(3^a)(3^2) - (3^a) = (3^2)(8)(3^27)
take (3^a) common on LHS
(3^a)[9-1] = (3^29)(8)
cancel 8 on both sides
(3^a) = (3^29)
a= 29
2a = 58
option E
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Re: If 3^a – 3^(a – 2) = 8(3^27), what is the value of 2a ?  [#permalink]

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Bunuel wrote:
If $$3^a – 3^{(a – 2)} = 8(3^{27})$$, what is the value of 2a ?

A. 20
B. 25
C. 27
D. 29
E. 58

LHS: $$3^a-\frac{3^a}{3^2}$$ $$= 3^a(1-\frac{1}{3^2})$$ = $$3^a*\frac{8}{9}$$
or, $$3^{a-2}*8$$ $$= 8*3^{27}$$
Hence $$a-2=27$$ or $$a=29$$. Therefore $$2a = 58$$

Option E
GMAT Club Legend  V
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Re: If 3^a – 3^(a – 2) = 8(3^27), what is the value of 2a ?  [#permalink]

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Top Contributor
Bunuel wrote:
If $$3^a – 3^{(a – 2)} = 8(3^{27})$$, what is the value of 2a ?

A. 20
B. 25
C. 27
D. 29
E. 58

Given: $$3^a – 3^{(a – 2)} = 8(3^{27})$$

Factor $$3^{(a – 2)}$$ from the left side to get: $$3^{(a – 2)}(3^2 – 1) = 8(3^{27})$$

Evaluate the part in the brackets: $$3^{(a – 2)}(8) = 8(3^{27})$$

Divide both sides by 8 to get: $$3^{(a – 2)} = 3^{27}$$

This means: $$a - 2 = 27$$

Solve: a = 29
So, 2a = (2)(29) = 58

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Re: If 3^a – 3^(a – 2) = 8(3^27), what is the value of 2a ?  [#permalink]

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Bunuel wrote:
If $$3^a – 3^{(a – 2)} = 8(3^{27})$$, what is the value of 2a ?

A. 20
B. 25
C. 27
D. 29
E. 58

We can simplify the given equation:

3^a - (3^a)(3^-2) = 8(3^27)

Factor 3^a from the left side:

3^a(1 - 1/9) = 8(3^27)

3^a(8/9) = 8(3^27)

3^a = 8(3^27) x 9/8

3^a = 3^27 x 3^2

3^a = 2^29

a = 29

So, 2a = 58.

Alternate Solution:

Observe that 3^a = 3^(a - 2 + 2) = (3^(a - 2))(3^2). Then:

(3^(a - 2))(3^2) - (3^(a - 2)) = 8(3^27)

Let’s factor the common 3^(a - 2) from the left side:

3^(a - 2)(3^2 - 1) = 8(3^27)

3^(a - 2)(8) = 8(3^27)

3^(a - 2) = 3^27

a - 2 = 27

a = 29

2a = 58

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If 3^a – 3^(a – 2) = 8(3^27), what is the value of 2a ?  [#permalink]

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RHS
8 can be written as (9-1) or (3²-1)
8(3²⁷)=(3²-1)X(3²⁷)
=3²⁹-3²⁷
let 2a=10 or a=5
plugin into LHS and check
3⁵-3³-Not correct
let 2a=58
a=29
Plugin to LHS of the equation
3²⁹-3²⁷=LHS=correct=E

Originally posted by gps5441 on 24 Sep 2017, 06:27.
Last edited by gps5441 on 24 Sep 2017, 06:30, edited 1 time in total.
Manager  B
Joined: 04 May 2014
Posts: 151
Location: India
WE: Sales (Mutual Funds and Brokerage)
Re: If 3^a – 3^(a – 2) = 8(3^27), what is the value of 2a ?  [#permalink]

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RHS
8(3²⁷)=(3²-1)X(3²⁷)
=3²⁹-3²⁷
let 2a=10 or a=5
plugin into LHS and check
3⁵-3³-Not correct
let 2a=58
a=29
Plugin to LHS of the equation
3²⁹-3²⁷=LHS=correct=E
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Status: QA & VA Forum Moderator
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Posts: 4772
Location: India
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WE: Business Development (Commercial Banking)
Re: If 3^a – 3^(a – 2) = 8(3^27), what is the value of 2a ?  [#permalink]

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1
Bunuel wrote:
If $$3^a – 3^{(a – 2)} = 8(3^{27})$$, what is the value of 2a ?

A. 20
B. 25
C. 27
D. 29
E. 58

Or, $$3^a – 3^{(a – 2)} = 8(3^{27})$$

Or, $$3^a – \frac{3^a}{3^2} = (3^2 - 1)(3^{27})$$

Or, $$3^a*3^2 – 3^a = 3^2(3^2 - 1)(3^{27})$$

Or, $$3^a(3^2 – 1) = 3^2(3^2 - 1)(3^{27})$$

Or, $$3^a = 3^2(3^{27})$$

Or, $$3^a = 3^{29}$$

Or, $$a = 29$$

So, 2a = 58 , answer will be (E)

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Re: If 3^a – 3^(a – 2) = 8(3^27), what is the value of 2a ?  [#permalink]

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_________________ Re: If 3^a – 3^(a – 2) = 8(3^27), what is the value of 2a ?   [#permalink] 19 Jan 2019, 23:30
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