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What is the greatest prime factor of 4^17 - 2^28?

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Re: What is the greatest prime factor of 4^17 - 2^28? [#permalink]

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New post 14 Mar 2016, 08:30
student26 wrote:
What is the greatest prime factor of 4^17 - 2^28?

A. 2
B. 3
C. 5
D. 7
E. 11

It can be written as 2^34 - 2^28 = 2^28*(2^6 - 1) = 2^28*63 = 2^28 * 3^2 * 7
Therefore, the answer is 7

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Re: What is the greatest prime factor of 4^17 - 2^28? [#permalink]

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New post 16 Mar 2016, 02:36
Here we can write 4^17 as 2^34
now taking 2^28 as factor the expression reduces to => 2^28 x 63
clearly 7 is the largest prime factor
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Re: What is the greatest prime factor of 4^17 - 2^28? [#permalink]

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New post 19 Apr 2016, 18:17
Flexxice wrote:
What is the greatest prime factor of 4^17 - 2^28?

A) 2

B) 3

C) 5

D) 7

E) 11



First of all 4^17 = (2^2)^17 = 2^34

So, 4^17 - 2^28 = 2^34 - 2^28
= 2^28(2^6 - 1) [factored out 2^28]
= 2^28(2^3 + 1)(2^3 - 1) [factored again]
= 2^28(8 + 1)(8 - 1) [evaluated]
= 2^28(9)(7) [evaluated]
= 2^28(3)(3)(7)

So, as you can see, the greatest prime factor is 7

Answer = D

Cheers,
Brent
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Re: What is the greatest prime factor of 4^17 - 2^28? [#permalink]

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New post 19 Apr 2016, 21:21
GMATPrepNow wrote:
Flexxice wrote:
What is the greatest prime factor of 4^17 - 2^28?

A) 2

B) 3

C) 5

D) 7

E) 11



First of all 4^17 = (2^2)^17 = 2^34

So, 4^17 - 2^28 = 2^34 - 2^28
= 2^28(2^6 - 1) [factored out 2^28]
= 2^28(2^3 + 1)(2^3 - 1) [factored again]
= 2^28(8 + 1)(8 - 1) [evaluated]
= 2^28(9)(7) [evaluated]
= 2^28(3)(3)(7)

So, as you can see, the greatest prime factor is 7

Answer = D

Cheers,
Brent



Thank you! I tried it in another way and I do not get why I got another answer than you did.

4^17 - 2^28 = 4^17 - 4^14 = 4^3 = 2^6

Because of that, I chose answer A.

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What is the greatest prime factor of 4^17 - 2^28? [#permalink]

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New post 19 Apr 2016, 22:04
Flexxice wrote:
GMATPrepNow wrote:
Flexxice wrote:
What is the greatest prime factor of 4^17 - 2^28?

A) 2

B) 3

C) 5

D) 7

E) 11



First of all 4^17 = (2^2)^17 = 2^34

So, 4^17 - 2^28 = 2^34 - 2^28
= 2^28(2^6 - 1) [factored out 2^28]
= 2^28(2^3 + 1)(2^3 - 1) [factored again]
= 2^28(8 + 1)(8 - 1) [evaluated]
= 2^28(9)(7) [evaluated]
= 2^28(3)(3)(7)

So, as you can see, the greatest prime factor is 7

Answer = D

Cheers,
Brent



Thank you! I tried it in another way and I do not get why I got another answer than you did.

4^17 - 2^28 = 4^17 - 4^14 = 4^3 = 2^6

Because of that, I chose answer A.


Hi,
you are wrong in the highlighted portion--
\(\frac{4^{17}}{4^{14}}= 4^{17-14} =4^3\)
\(4^{17} - 4^{14} = 4^{14}(4^3-1) = 4^{14} *(64-1) = 4^{14}*63= 4^{14}*7*9..\)
so 7 is the answer..
You can not subtract the way you have done, it can be done ONLY if two terms are being divided as shown above..

a simpler examplewill be
\(2^3 -2^1\).. as per you it will be\(2^2=4.\).
BUT \(2^3-2^1=8-2=6..\)
hope it helps
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Re: What is the greatest prime factor of 4^17 - 2^28? [#permalink]

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New post 21 Apr 2016, 01:57
Flexxice wrote:


Thank you! I tried it in another way and I do not get why I got another answer than you did.

4^17 - 2^28 = 4^17 - 4^14 = 4^3 = 2^6

Because of that, I chose answer A.


We cannot simply subtract the terms.

4^17 - 4^14 = 4^14 (4^3 - 1) = 4^14*(64 - 1) = 4^14 * 63
Here we have taken 4^14 common from both the terms and written the remainder inside the brackets.
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Re: What is the greatest prime factor of 4^17 - 2^28? [#permalink]

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New post 15 Mar 2017, 17:40
Bunuel wrote:
student26 wrote:
What is the greatest prime factor of 4^17 - 2^28?

A. 2
B. 3
C. 5
D. 7
E. 11


\(4^{17}-2^{28}=2^{34}-2^{28}=2^{28}(2^6-1)=2^{28}*63=2^{28}*3^2*7\) --> greatest prime factor is 7.

Answer: D.



Why is it (2^6-1)? :oops:

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Re: What is the greatest prime factor of 4^17 - 2^28? [#permalink]

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New post 16 Mar 2017, 07:07
Aline2289 wrote:
Bunuel wrote:
student26 wrote:
What is the greatest prime factor of 4^17 - 2^28?

A. 2
B. 3
C. 5
D. 7
E. 11


\(4^{17}-2^{28}=2^{34}-2^{28}=2^{28}(2^6-1)=2^{28}*63=2^{28}*3^2*7\) --> greatest prime factor is 7.

Answer: D.



Why is it (2^6-1)? :oops:


Added extra step:

\(4^{17}-2^{28}=2^{34}-2^{28}=2^{28}*2^6-2^{28}=2^{28}(2^6-1)=2^{28}*63=2^{28}*3^2*7\) --> greatest prime factor is 7.

So, we are factoring out \(2^{28}\) from \(2^{34}-2^{28}\).

Hope it's clear.
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Re: What is the greatest prime factor of 4^17 - 2^28? [#permalink]

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student26 wrote:
What is the greatest prime factor of 4^17 - 2^28?

A. 2
B. 3
C. 5
D. 7
E. 11


We need to determine the greatest prime factor of 4^17 – 2^28. We can start by breaking 4^17 into prime factors.

4^17 = (2^2)^17 = 2^34

Now our equation is as follows:

2^34 – 2^28

Note that the common factor in each term is 2^28; thus, the expression can be simplified as follows:

2^28(2^6 – 1)

2^28(64 – 1)

2^28(63)

2^28 x 9 x 7

2^28 x 3^2 x 7

We see that the greatest prime factor must be 7.

Answer: D
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Re: What is the greatest prime factor of 4^17 - 2^28? [#permalink]

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New post 27 Aug 2017, 08:52
2^34-2^28=2^28(2^6 -1)
=2^28(64-1)
=2^28(9*7)
Hence largest prime factor 7

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Re: What is the greatest prime factor of 4^17 - 2^28?   [#permalink] 27 Aug 2017, 08:52

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