Where's the problem statement?

Algebra Level 3

The answer to the following question is an integer, which can also be expressed as a b \frac{a}{b} , where a a and b b are positive integers.

What is 2 a 2 14 a b + 7 ? 2a^2-14ab+7?


Hint: If you aren't familiar with self-referential puzzles, you can try this similar problem .


The answer is 7.

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5 solutions

Steven Yuan
Jul 14, 2017

Notice that the answer to this problem is simultaneously 2 a 2 14 a b + 7 2a^2 - 14ab + 7 and a b . \frac{a}{b}. Equating the two and doing algebraic manipulation yields

2 a 2 14 a b + 7 = a b 2 a 2 b 14 a b 2 a + 7 b = 0 2 a b ( a 7 b ) ( a 7 b ) = 0 ( 2 a b 1 ) ( a 7 b ) = 0. \begin{aligned} 2a^2 - 14ab + 7 &= \dfrac{a}{b} \\ 2a^2b - 14ab^2 - a + 7b &= 0 \\ 2ab(a - 7b) - (a - 7b) &= 0 \\ (2ab - 1)(a - 7b) &= 0. \end{aligned}

Thus, either 2 a b 1 = 0 2ab - 1 = 0 or a 7 b = 0. a - 7b = 0. The first equality gives a b = 1 2 , ab = \frac{1}{2}, which can never be satisfied, since a , b a, b are integers. Therefore, we conclude that a = 7 b , a = 7b, so the answer to the problem is a b = 7 b b = 7 . \frac{a}{b} = \frac{7b}{b} = \boxed{7}.

I couldn't figure out what the question was! Maybe better phrased as "2a^2 - 14ab + 7 = a/b where a and b are integers, what is a/b?"

Hugh Bothwell - 3 years, 10 months ago

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The main idea of the problem is to figure out what the problem was asking. If you're from the weekly problems and couldn't read the title of the problem, it is "Where's the problem statement?" Simply stating the equality destroys the spirit of the problem.

Steven Yuan - 3 years, 10 months ago

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I also think the problem is confusingly stated, and I don't agree that that adds to it somehow. Given how oddly you'd chosen to state it, I spent a while thinking that the a/b in the first sentence weren't meant to be the same variables as the equation, and that you were simply letting us know the answer is rational.

Timothy Smith - 3 years, 10 months ago

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@Timothy Smith I noticed that some of the most controversial problems on Brilliant are these sorts of self-referential questions. I suggest submitting a report on this problem and having a mod deal with your concern.

Steven Yuan - 3 years, 10 months ago

How is "figuring out what the author meant?" a math problem? To my eyes it looked like part of the problem had been omitted. That's been known to happen.

Richard Desper - 3 years, 10 months ago

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@Richard Desper Not all math problems are straight-on math. Many problems do, in fact, require careful reading and deciphering of what the problem is actually asking. The American Invitation Mathematics Exam (AIME) does this a lot: the later questions on the AIME often contain scary-looking notation or long-winded descriptions to make the question seem harder than it actually is. It's also not uncommon to see self-referential questions on sprint rounds in math tournaments.

But, of course, what is clever to one person might be confusing to another. If enough people complain, then I suggest submitting a report to change the wording of the problem to make it clearer.

Steven Yuan - 3 years, 10 months ago

I think what made this question more confusing was that the statement was written in the same style as other problems where it is just telling you the format of what you type (e.g. you input 2/3 to the text box). But I do agree that having the title would help a bit.

Amanda Kirchgraber - 3 years, 10 months ago

Ya. Even i was confused when the 1st time I read the question. Nice problem

Md Zuhair - 3 years, 11 months ago

I would have written the problem as follows:

The answer to the question below can be written as a b \dfrac{a}{b} where a a and b b are positive integers.

What is 2 a 2 14 b + 7 2a^2 - 14b + 7 ?

Rocco Dalto - 3 years, 10 months ago

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Whats the difference sir?

Md Zuhair - 3 years, 10 months ago

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It is clearer to me. It refers to the question below. Stating "this question" was somewhat confusing at first.

Rocco Dalto - 3 years, 10 months ago

The question has been edited.

Pi Han Goh - 3 years, 10 months ago

Is that what the question was?

Richard Desper - 3 years, 10 months ago

What did you do on those third and forth steps? That make no sense to me.

Dog Marquetet - 3 years, 10 months ago

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Nevermind it makes sense now

Dog Marquetet - 3 years, 10 months ago

I don't understand the part where you say a/b cannot be 1/2, could you explain it? Thanks

Utsav Garg - 3 years, 10 months ago

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The equality was a b = 1 2 , ab = \frac{1}{2}, and it can't be satisfied because the product of two integers is always another integer and cannot be an irreducible fraction.

Steven Yuan - 3 years, 10 months ago

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Thanks, I get it now!

Utsav Garg - 3 years, 10 months ago

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@Utsav Garg No problem! ¨ \ddot \smile

Steven Yuan - 3 years, 10 months ago

FYI A more interesting version of this problem would arise when gcd ( a , b ) 1 \gcd(a,b) \neq 1 .

Calvin Lin Staff - 3 years, 10 months ago

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Ya, very true. It will be quite tough

Md Zuhair - 3 years, 10 months ago

What do you mean? a=14 and b=2 => gcd(14,2) = 2 and still leads to a/b=7.

P S - 2 years, 7 months ago

I'm having trouble finding an error in this process: when a and b are positive integers, (2a^2)-(14ab)+7 should equal an integer and also equals a/b. So a/b should be an integer. Let's say a=xb where x is an integer. Now substitute all "a"s with xb:

(2x^2b^2)-(14xb^2)+7 = xb/b Move 7 to RHS and factor: 2b^2x(x-7) = x-7 Divide both sides by x-7: 2b^2x = 1 (b^2)(x) = 1/2 or ab = 1 should not be possible, as and b are positive integers.

Joseph Wilson - 3 years, 10 months ago

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Your process is correct except in the case where x 7 = 0. x - 7 = 0.

Steven Yuan - 3 years, 10 months ago
Peter Chan
Jul 25, 2017

I think the question was written with inappropriate wording that would give rise to misinterpretation. There is no way to solve this hadn't I been able to interpret the question as follows:

Given that and a, b are positive integers. What is the value of 2 a 2 14 a b + 7 2a^{2}-14ab+7 if the value is also equal to a b \frac{a}{b} ?

In which case it is 7

Yes. The formula isn't presented as an equation, so the answer is whatever you get when you plug in your choice of A and B. Phrasing! We can't give the answer you're looking for if you don't clearly ask the correct question.

Ban Yan - 3 years, 10 months ago

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Is the phrasing better now?

Pi Han Goh - 3 years, 10 months ago

@Ban Yan and @Peter Chan , the point of the question is to realize that it is self-referential. It's meant to be confusing on the surface until you really read into its details and understand that the question refers to itself and the answer lies therein.

Zach Abueg - 3 years, 10 months ago

Type it, no spaces around the symbols, as \ ( 2a ^ { 2 } - 14ab + 7 \ )

Avery Bentley Sollmann - 3 years, 10 months ago

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Thanks Avery !

Peter Chan - 3 years, 10 months ago
Rocco Dalto
Jul 24, 2017

2 a 2 14 a b + 7 = a b 2 a 2 b 14 a b 2 + 7 b a = 0 2a^2 - 14ab + 7 = \dfrac{a}{b} \implies 2a^2b - 14ab^2 + 7b - a = 0 \implies

2 a 2 b a 14 a b 2 + 7 b = 0 a ( 2 a b 1 ) 7 b ( 2 a b 1 ) = 0 ( a 7 b ) ( 2 a b 1 ) = 0 2a^2b - a - 14ab^2 + 7b = 0 \implies a(2 ab - 1) - 7b(2ab - 1) = 0 \implies (a - 7b)(2ab - 1) = 0 \implies

a 7 b = 0 a - 7b = 0 or 2 a b 1 = 0 2ab - 1 = 0

2 a b 1 = 0 a b = 1 2 2ab - 1 = 0 \implies ab = \dfrac{1}{2} which has no solution for positive integers a a and b b .

a = 7 b a b = 7. \therefore a = 7b \implies \dfrac{a}{b} = 7.

On LHS is a/b. When b is being multiplied with the equation on LHS , how LHS became 0. Wt happened to 'a'. Y it didn't get on the other side to divide equation. Help needed. Anyone plz. Thanx in advance.

Asima jarral - 3 years, 10 months ago

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The b was multiplied first, leaving the RHS as only a. a was then subtracted. That's where the -a term comes from.

Phillip Temple - 3 years, 10 months ago

An elegant puzzle, no doubt!

Brian Whatcott - 3 years, 10 months ago
James Moors
Jul 29, 2017

The answer to the following question is an integer , which can also be expressed as a b \frac{a}{b} . By this, b = 1 b=1 .

The problem then reduces to: 2 a 2 14 a + 7 = a 2 a 2 15 a + 7 = 0 ( 2 a 1 ) ( a 7 ) = 0 a = 1 2 or a = 7 \begin{aligned} 2a^2-14a+7=a\\ 2a^2-15a+7=0\\ (2a-1)(a-7)=0\\ a = \frac{1}{2} \text{ or } a = 7 \end{aligned}

Since only one solution is an integer a = 7 a = \boxed{7} .

Ayon Ghosh
Jul 25, 2017

nice one !

2 a 2 2a^2 - 14 a b 14ab + + 7 7 = = a a / / b b

(this is the problem statement !)

cross multplying and rearranging we get

2 a b 2ab ( a 7 b ) (a - 7b) = = ( a 7 b ) (a - 7b)

this statement gives 2 2 possibilities,

either a b ab = = 1 / 2 1/2 ( the condition with this is a not equal to 7b)

O R OR

a a = = 7 b 7b

but the former statement is not possible since ( a , b ) (a,b) are positive integers.

so a = 7 b a = 7b so a / b a/b = = 7 7 .

this was very simple it was just the basic concept

GURWINDER SINGH - 3 years, 10 months ago

@GURWINDER SINGH yes i agree.

Ayon Ghosh - 3 years, 10 months ago

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