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# x^6 = y^2 + 127

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x^6 = y^2 + 127 [#permalink]  11 Dec 2009, 22:23
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0% (00:00) correct 100% (00:47) wrong based on 1 sessions
Find the number of pairs of positive integers (x, y) such that x^6 = y^2 + 127.

(A) 0
(B) 1
(C) 2
(D) 3
(E) 4

pls share if u used a quick approach to solve this
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Re: x^6 = y^2 + 127 [#permalink]  12 Dec 2009, 00:06
xcusemeplz2009 wrote:
Find the number of pairs of positive integers (x, y) such that x^6 = y^2 + 127.

(A) 0
(B) 1
(C) 2
(D) 3
(E) 4

pls share if u used a quick approach to solve this

First note that: x and y are +ve integers i.e. x and y cannot be -ves, zeros and fractions.

Given that:- x^6 = y^2 + 127
x can have a value of 1....................n, however x>2 because:

If x = 1, x^6 = 1 and y = sqrt(1-127). y is an irrational number. Not possible.
If x = 2, x^6 = 64 and y = sqrt(64-127). y is an irrational number. Not possible.
If x = 3, x^6 = 729 and y = sqrt(729-127) = sqrt(602). Now y is a fraction. Not possible.
If x = 4, x^6 = 64x64 and y = sqrt{(64x64) -127} = .......?

Now its not possible to do any calculation beyond this point in 2-5 minuets. Until and unless there is a quick approach, I would say this not a gmat-type question. I would love to see if anybody has a quick approach.

I thought its a good question however it turned to be a tough one.
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Re: x^6 = y^2 + 127 [#permalink]  12 Dec 2009, 01:48
Thats correct, even I used brute force. But can we find some better approach for similar question, where constant may be changed i.e. 64 or exponent may be changed. ?
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Re: x^6 = y^2 + 127 [#permalink]  12 Dec 2009, 07:16
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HI.. a quick appch i can think of..
the eq can be written as... x^6-y^2=127...x^6-y^2=(x^3-y)(x^3+y)....which means 127 is product of two int... but if u look at 127..it is prime number... so there is no pair of positive integers which satisfies the condition ...ANS-0
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Re: x^6 = y^2 + 127 [#permalink]  12 Dec 2009, 08:03
1
KUDOS
xcusemeplz2009 wrote:
Find the number of pairs of positive integers (x, y) such that x^6 = y^2 + 127.

(A) 0
(B) 1
(C) 2
(D) 3
(E) 4

pls share if u used a quick approach to solve this

chetan2u's approach above is excellent, though there's one problem with the analysis. We have

(x^3 + y)(x^3 - y) = 127

so x^3 + y and x^3 - y must be factors of 127. Since 127 is prime, the only possibility is that x^3 + y = 127 and x^3 - y = 1. Now, x cannot be greater than 5, since that would make x^3 larger than 127, and since x^3 must be greater than y, x could only be 4 or 5. Still testing these values, we do find that x = 4 and y = 63 gives a legitimate solution here, so there is one pair of values that works.
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Re: x^6 = y^2 + 127 [#permalink]  12 Dec 2009, 08:06
chetan2u wrote:
HI.. a quick appch i can think of..
the eq can be written as... x^6-y^2=127...x^6-y^2=(x^3-y)(x^3+y)....which means 127 is product of two int... but if u look at 127..it is prime number... so there is no pair of positive integers which satisfies the condition ...ANS-0

This is definitely a tough one... but I do not think the answer is 0.

chetan2u had a great approach, same as the one I used but since 127 is a prime number I went a step further:
(x^3-y)(x^3+y)=127
(x^3-y)(x^3+y)=(1)(127)

From here I plugged in different values of x and y to see if I can get (x^3-y) to equal 1 and (x^3+y) to equal 127. Because we know that (x^3-y) must equal 1, y must be x^3-1.

x=1, not possible
x=2, not possible
x=3, not possible

x=4, y=63
(x^3-y)(x^3+y)=127
(4^3-63)*(4^3+63)=127

x=5, not possible

Can someone please verify the OA?
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Re: x^6 = y^2 + 127 [#permalink]  12 Dec 2009, 08:25
hi ianstewart...
thanks a lot... should not have skipped my mind but it seems in a hurry, just overlooked it...
lesson learnt-give a thought before proceeding to next step..
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Re: x^6 = y^2 + 127 [#permalink]  12 Dec 2009, 13:10
chetan2u wrote:
HI.. a quick appch i can think of..
the eq can be written as... x^6-y^2=127...x^6-y^2=(x^3-y)(x^3+y)....which means 127 is product of two int... but if u look at 127..it is prime number... so there is no pair of positive integers which satisfies the condition ...ANS-0

Thats a good point.

x^6-y^2 = 127
(x^3-y)(x^3+y) = 127 x 1

Since x and y both are +ves,
(x^3-y) = 1 and
(x^3+y) = 127

As I mentioned earlier, x >2. Now the possibilities for x are <5. So x could be 3 or 4 or 5.

After few trails, x = 4.

Agree with OA as B.
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Re: x^6 = y^2 + 127 [#permalink]  12 Dec 2009, 15:21
I believe, solving these kinds of questions is key to scoring well on the test. When we never know which question is experimental, a tough one as worse as this can always waste time and lower scores. I have had similar questions and I did not have sufficient practise facing them and lost valuable time. The key is to not panic and try to think at least one step beyond or differently for every 3 to 5 seconds when facing a tough problem.
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Re: x^6 = y^2 + 127 [#permalink]  21 Oct 2011, 03:34
2
KUDOS
xcusemeplz2009 wrote:
Find the number of pairs of positive integers (x, y) such that x^6 = y^2 + 127.

(A) 0
(B) 1
(C) 2
(D) 3
(E) 4

pls share if u used a quick approach to solve this

Here is what I would do:

First thing that comes to my mind is that 127 is prime. Well then, I can't do anything with it right now.
Then I see that there are some squares
(x^3)^2 - y^2 = 127
Let's sayx^3 = a

a^2 - y^2 = 127
(a+y)(a-y) = 127*1
We know that a and y are both positive integers. Therefore, their sum, a+y = 127 and their difference, a-y = 1.
It is obvious that a and y must be 64 and 63. (or you can solve the two equations simultaneously to get the values for a and y)
If a = 64 = x^3, x must be 4.
So there is only one pair of values (4, 63).

The question is straight forward because 127 is prime. You get only one pair of values. If instead, we have a composite number with many factors, we need to find the possible values of a and y and then see which values of a work for us.
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Re: x^6 = y^2 + 127   [#permalink] 21 Oct 2011, 03:34
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