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Re: A right triangle has sides of a, b, and 11, respectively, [#permalink]

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13 Jul 2014, 00:25

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Here is the explanation -

Having sides a, b, and 11 is a RIGHT angle triangle , Hence using Pythagoras theorem - CASE 1 -> a*a + b*b = 11*11 (Assuming 11 is the longest side) or CASE 2 -> 11*11 + b*b = a*a (Assuming a is the longest side)

Do not forget that we are also given all sides are integer.

Consider CASE 1 now - Can you think of any set of two integers (both lest than 11 ) such as a*a + b*b = 11*11 = 121

we have perfect squares less than 121 as - 1, 4, 9, 16, 25, 36, 49, 64, 81, 100 If you add any of the above two you will never get sum as 121 . Hence CASE 1 is VOID

As we are given All sides are integer -> a, b are integers -> 'a+b' and 'a-b' are also integers. Equation 1 states that 121 is [color=#0072bc]product of two integers and 121 is a very interesting number in the way that it has only two combination of product of two integers Combination 1 : a+b = 11 also a-b = 11 -> This case is invalid as it will result that one side of triangle b =0 Combination 2 : a+b = 121 and a-b=1 -> This case is Valid and gives us the answer that 'a+b' = 121 [/color]

Hence option E Tricky question indeed !
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Re: A right triangle has sides of a, b, and 11, respectively, [#permalink]

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13 Jul 2014, 00:43

A pythagorean triplet involving 11 is (11,60,61) . Since, the other two sides are integers, hence this satisfies. and we have 60+61=121 in the option (Although,I do not advocate the usage of this fundamental in solving the questions , but if one is through with basic pythagorean triplets then it would really help in saving some valuable time)

It is debatable if a three sided figure with one zero length is a triangle. Is this question GMAT worthy?

The length must be greater than 0. The sides are {11, 60, 61}: each sides is greater than 0. Don't see any problem with this question.
_________________

Since the first two right triangles with integer sides are the 3-4-5, the 6-8-10, and the 5-12-13, 11 cannot be the hypotenuse. Hence 11 is one of the legs, and our equation is \(11^2 + a^2 = b^2.\) Subtracting a^2 from both sides gives us \(11^2 = b^2 - a^2\), or \(121 = (b + a)(b - a)\). Since b and a are both positive integers, (b + a) must = 121 and (b - a) must = 1. (The only other option, (b+a)=(b-a)=11 is impossible with positive a.) Hence \((b + a) = 121\).

Last edited by Bunuel on 07 Jul 2015, 11:39, edited 1 time in total.

Re: A right triangle has sides of a, b, and 11, respectively, [#permalink]

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07 Jul 2015, 09:02

I did not understand the Veritas official explanation for this, they have taken the standard ratios - 3-4-5, 5-12-13 and determine on the basis of those, that the hyp cannot be 11, could someone explain this reasoning?

A right triangle has sides of a, b, and 11, respectively, where a and b are both integers. What is the value of (a + b)?

A. 15 B. 57 C. 93 D. 109 E. 121

The trick for this question is

If the side of a right angle triangle is a prime number then other two sides will be

Second Side = [(Prime)^2 + 1]/2

Third Side = [(Prime)^2 - 1]/2

i.e. for One side = 11

Second Side = (11^2 -1)/2 = (121-1)/2 = 60 Third Side = (11^2 +1)/2 = (121+1)/2 = 61

I hope this helps!

I personally find it an UNSUITABLE question for GMAT... GMAT Doesn't expect such tricks from students.
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If the side of a right angle triangle is a prime number then other two sides will be

Second Side = [(Prime)^2 + 1]/2

Third Side = [(Prime)^2 - 1]/2

First, if all you know is that the length of one side is prime, there are infinitely many possible lengths for the other sides. It's only if you know the other sides are integers that there might be a formula.

But you also mean to say "if the shortest side of a right triangle is prime, and the other sides are of integer length, the other sides will be..." If all you know is that one side is of length 5, say, using your formulas you'd think the other sides need to be 12 and 13. They could also be 3 and 4.

GMATinsight wrote:

I personally find it an UNSUITABLE question for GMAT... GMAT Doesn't expect such tricks from students.

No one will ever need the formulas you posted in a GMAT question, and it would be pointless for a test taker to memorize them. But you certainly don't need those formulas here. You only need to know the difference of squares factorization, and that there is only one way to write 11^2 as a product of two distinct positive integers. The question is well within the scope of the GMAT.
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If you are looking for online GMAT math tutoring, or if you are interested in buying my advanced Quant books and problem sets, please contact me at ianstewartgmat at gmail.com

If the side of a right angle triangle is a prime number then other two sides will be

Second Side = [(Prime)^2 + 1]/2

Third Side = [(Prime)^2 - 1]/2

First, if all you know is that the length of one side is prime, there are infinitely many possible lengths for the other sides. It's only if you know the other sides are integers that there might be a formula.

But you also mean to say "if the shortest side of a right triangle is prime, and the other sides are of integer length, the other sides will be..." If all you know is that one side is of length 5, say, using your formulas you'd think the other sides need to be 12 and 13. They could also be 3 and 4.

GMATinsight wrote:

I personally find it an UNSUITABLE question for GMAT... GMAT Doesn't expect such tricks from students.

No one will ever need the formulas you posted in a GMAT question, and it would be pointless for a test taker to memorize them. But you certainly don't need those formulas here. You only need to know the difference of squares factorization, and that there is only one way to write 11^2 as a product of two distinct positive integers. The question is well within the scope of the GMAT.

You seem of have less understanding of the word "OPINION"

I hope you prove me wrong by bringing ONE question from authentic source of GMAT questions if you have enough exposure of GMAT.(Refer OG, GMAT PREP etc.)

Another point, whether you take (3, 4, 5) or (5, 12, 13) it fit in both cases. and the forward application doesn't approve of backward application as well. (1 is a natural no. doesn't mean that a natural no. has to be 1)

And yes, It fit's to the cases when the smallest side is prime Number and the all Sides are of Integer length so I must admit you are good at finding faults and not as good in appreciating a new concept and an opinion.
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Another point, whether you take (3, 4, 5) or (5, 12, 13) it fit in both cases. and the forward application doesn't approve of backward application as well. (1 is a natural no. doesn't mean that a natural no. has to be 1)

I'm afraid I don't understand your reply. In your first post, you said that when "the side of a right angle triangle is a prime number then other two sides will be" x and y. That is, you said we can determine the other two sides from one side, when the length of that one side is prime. That's not true, but if a test taker believed it was, he or she might answer questions incorrectly on the GMAT - for example, a DS question like the following:

If the lengths of the three sides of a right triangle are integers, what is the perimeter of the triangle? 1. The length of one side is 5 2. Two of the lengths are prime numbers

A test taker who thinks that a single prime length is sufficient to determine the other two will think Statement 1 is sufficient, and will pick A, which is not the right answer - the answer is E, since the triangle might be a 3-4-5 or 5-12-13 triangle. I will always post a clarification if an expert here posts something which might be misinterpreted, and which might lead a test taker to answer test questions incorrectly, and I'd always be grateful to anyone who posts a clarification of one of my posts if I were ever to make an error.

I also don't understand why you think I confused your statement with its converse (what you're suggesting in your comment about natural numbers). I did not. Nor do I understand why you consider it relevant that the 3-4-5 triangle fits your formula, since we don't find the lengths 3 and 4 by plugging in '5', but regardless there are other triangles with a prime length that do not satisfy those formulas - the 8-15-17 triangle, for example.

GMATinsight wrote:

You seem of have less understanding of the word "OPINION"

I hope you prove me wrong by bringing ONE question from authentic source of GMAT questions if you have enough exposure of GMAT.(Refer OG, GMAT PREP etc.)

I don't think it often is a matter of opinion whether a question falls within the scope of the GMAT. It's not clear to me precisely why you think this question is out of the scope of the test. I agree with you when you say, referring to your formulas, that "GMAT Doesn't expect such tricks from students." That seemed to me the only reason you described this question as "unsuitable" for the test. But those formulas aren't at all necessary here, so I disagreed with your initial premise. The main mathematical issue in this question, when solving it using the difference of squares factorization, is recognizing that when b > c > 0, and b and c are positive integers, the equation (b+c)(b-c) = 121 has only one possible solution. If you want to see an official question that relies on very similar logic, see this one from GMATFocus:

If you are looking for online GMAT math tutoring, or if you are interested in buying my advanced Quant books and problem sets, please contact me at ianstewartgmat at gmail.com

Re: A right triangle has sides of a, b, and 11, respectively, [#permalink]

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29 Aug 2016, 02:36

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Re: A right triangle has sides of a, b, and 11, respectively, [#permalink]

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01 Nov 2016, 15:48

GMATinsight wrote:

IanStewart wrote:

GMATinsight wrote:

The trick for this question is

If the side of a right angle triangle is a prime number then other two sides will be

Second Side = [(Prime)^2 + 1]/2

Third Side = [(Prime)^2 - 1]/2

First, if all you know is that the length of one side is prime, there are infinitely many possible lengths for the other sides. It's only if you know the other sides are integers that there might be a formula.

But you also mean to say "if the shortest side of a right triangle is prime, and the other sides are of integer length, the other sides will be..." If all you know is that one side is of length 5, say, using your formulas you'd think the other sides need to be 12 and 13. They could also be 3 and 4.

GMATinsight wrote:

I personally find it an UNSUITABLE question for GMAT... GMAT Doesn't expect such tricks from students.

No one will ever need the formulas you posted in a GMAT question, and it would be pointless for a test taker to memorize them. But you certainly don't need those formulas here. You only need to know the difference of squares factorization, and that there is only one way to write 11^2 as a product of two distinct positive integers. The question is well within the scope of the GMAT.

You seem of have less understanding of the word "OPINION"

I hope you prove me wrong by bringing ONE question from authentic source of GMAT questions if you have enough exposure of GMAT.(Refer OG, GMAT PREP etc.)

Another point, whether you take (3, 4, 5) or (5, 12, 13) it fit in both cases. and the forward application doesn't approve of backward application as well. (1 is a natural no. doesn't mean that a natural no. has to be 1)

And yes, It fit's to the cases when the smallest side is prime Number and the all Sides are of Integer length so I must admit you are good at finding faults and not as good in appreciating a new concept and an opinion.

Hi, could you then please elaborate a bit more on the solution proposed above? How can we reach the conclusion that 11 is not the hipotenuse?

Hi, could you then please elaborate a bit more on the solution proposed above? How can we reach the conclusion that 11 is not the hipotenuse?

The logic of "11 is not the hypotenuse" is quite simple. Note that all three sides of the right triangle are integers so the sides will form a pythagorean triplet. We know the first two pythagorean triplets are 3-4-5 and 5-12-13. If there were a pythagorean triplet such as a-b-11, we would know it as the second pythagorean triplet and 5-12-13 would be the third. So 11 will not be the hypotenuse but would be one of the legs. So a^2 + 11^2 = c^2 121 = (c+a)*(c-a) There are only 2 ways to factorize 121 into pairs of 2 factors each: (1 and 121) , (11, 11) Sum and difference of two integers cannot both be 11 so c+a = 121 and c-a = 1

Re: A right triangle has sides of a, b, and 11, respectively, [#permalink]

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02 Nov 2016, 00:44

VeritasPrepKarishma wrote:

bazu wrote:

Hi, could you then please elaborate a bit more on the solution proposed above? How can we reach the conclusion that 11 is not the hipotenuse?

The logic of "11 is not the hypotenuse" is quite simple. Note that all three sides of the right triangle are integers so the sides will form a pythagorean triplet. We know the first two pythagorean triplets are 3-4-5 and 5-12-13. If there were a pythagorean triplet such as a-b-11, we would know it as the second pythagorean triplet and 5-12-13 would be the third. So 11 will not be the hypotenuse but would be one of the legs. So a^2 + 11^2 = c^2 121 = (c+a)*(c-a) There are only 2 ways to factorize 121 into pairs of 2 factors each: (1 and 121) , (11, 11) Sum and difference of two integers cannot both be 11 so c+a = 121 and c-a = 1

Answer (E)

Thanks you are definitely a star. My knowledge of pythagorean triplet is a bit rusty, I didn't consider that if there is not a triplet (or a multiple) with 11 in the hypotenuse then 11 can't be the hypotenuse of a right triangle given the constraint that all the sides need to be integers

Re: A right triangle has sides of a, b, and 11, respectively, [#permalink]

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17 Jan 2017, 12:55

Its answer E

1. Check with recycled triangles 3:4:5/6:8:10/5:12:13 --> 11 cannot be the hypotenuse 2. 11 is one of the legs, so the equation is 11²+a²=b² ==> b²-a²=11² 3. Reminds me of recycled quadratic III (a+b)(a-b)=a²-b² and apply to above (works in 99% of cases) 4. (b+a)(b-a)=11²=121 / factorize 121 in factor tree leaves me only with 121 1, so (b+a) must be 121 since a and b are defined as integeres

Re: A right triangle has sides of a, b, and 11, respectively, [#permalink]

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23 Jan 2017, 13:26

anceer wrote:

A right triangle has sides of a, b, and 11, respectively, where a and b are both integers. What is the value of (a + b)?

A. 15 B. 57 C. 93 D. 109 E. 121

So if a and b both are integers, then a+b should also be integer. Now we are asked to find the value of (a+b)^2. I started POE. None of the values is a perfect square (which one square root result into a integer, which should be the value of base a+b) EXCEPT 121.

I am pretty sure it cant be that simple, what am i missing here