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Algebra Level 3

If A A is not equal to B B , which of the following is greater?

First: A A B Second: B A B \text{ First: } \frac{ A } {A-B} \quad \quad \text{ Second: } \frac{ B}{A-B}

First Second They are equal Not enough information

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49 solutions

A A B B A B = A B A B = 1 , \frac{A}{A-B} - \frac{B}{A-B} = \frac{A-B}{A-B} = 1, So, the first value always equals the second value plus one, thus the first value is greater.

what if (A-B ) is negative???

Vaibhav Agarwal - 7 years, 3 months ago

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It doesn't matter. The first equals the second plus one, which is greater than the second, thus the first is greater than the second.

Whitney Clark - 5 years, 7 months ago

its not what if .... indeed its negative a<b ∴ a-b<0

Pratik Pai - 6 years, 4 months ago

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you should mention it

vaibhav Chaturvedi - 5 years, 10 months ago

Vaibhav Agrwal If A-B IS NEGATIVE then result will still be correct

Amit Singh - 5 years, 6 months ago

A=1, B=0. A/(A-B) = 1, B/(A-B)=0 ?

Ng Donn - 7 years, 4 months ago

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Then the first is greater, but that doesn't mean it always works.

Whitney Clark - 5 years, 4 months ago

We can just simply put values for a and b and try like a=4 and b=3 :-)

karim ali - 7 years, 3 months ago

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that doesn't tell you that it's true for all values though.

Gaurav Dabas - 8 months ago

yes.. that's the easiest way

Rohit Udaiwal - 5 years, 9 months ago

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Rohit - It might be easy - but it could well be wrong, as all it does is prove the case for two specific values of A & B. In maths you have to be able to prove your answer for ALL possible values of A & B

Tony Flury - 5 years, 8 months ago

Take First-Second=1. Then the first should be greater :)

Carlos David Nexans - 6 years, 10 months ago

If I do this to B/(A-B) i will get it bigger

Saurabh Pranjale - 7 years, 4 months ago

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No.. try it.. the symmetry breaks because in the case you are talking about, we would not have B-A in the denominator. :-)

Snehal Shekatkar - 7 years, 4 months ago

Not at all, it will be A/A-B -1, don't you know maths

Kushagra Sahni - 7 years, 3 months ago

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exactly thus the first one is greater !

Rahul Deshpande - 7 years, 3 months ago

Nice solution!!

Prasun Biswas - 7 years, 4 months ago

Good Solution

Kushagra Sahni - 7 years, 3 months ago

A-B is always constant on the denominator they given A<B so 1<2

Manojkumar Susarla - 7 years, 3 months ago

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It is not given that A<B! Only A is not equal to B!

Мариана Кьосева - 5 years, 7 months ago

What if B/(A-B ) is negative?

Adhiraj Mandal - 7 years, 2 months ago

A not equal to B can mean, A > B which make this solution correct. if B > A then, second equation will greater than first one.

Karn Roswell - 7 years, 1 month ago

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Haha...., you did not check it. First is greater always.

Muhammad Abdullah - 5 years, 9 months ago

what if the A/(A-B) is a rational fraction??or negative??then B/(A-B) would be greater?

Manojit Ganguly - 6 years, 2 months ago

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If a<b then a-b<0 a/(a-b) is a negetive quantity ... Also b/(a-b)... But |a|<|b|.. Therefore b/(a-b) is lesser .

Saumik Karfa - 6 years, 2 months ago

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It is not given that |A| < |B| only that A and B are not equal.

David Kaziska - 5 years, 8 months ago

Nice solution!

Walter Tay - 5 years, 4 months ago

This is a lie, proof:

F = A/(A - B) S = B/(A - B)

Suppose F < S or S < F:

A/(A-B) < B/(A-B) A/(A-B) > B/(A-B)

Multiply by (A-B):

A < B B > A

Then: Not Enough Information!!!

Angel Raygoza - 1 year, 2 months ago

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In case of S<F, as your conclusion is that B>A, it implies that A-B<0.

Hence, at the point when you multiplied by (A-B), sign of inequality would have reversed.

Giri V K - 7 months, 3 weeks ago

this is nuts,wtf math?

Bruno Martel - 2 months, 2 weeks ago

Just take random values...

Tanya Gupta - 7 years, 4 months ago

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In general taking random values is a bad way to prove a mathematical statement - it maybe that you find a set of numbers that work - but it could be that there are still an infinite number of sets of numbers which fail.

Tony Flury - 5 years, 6 months ago

Won't work for negative

chinmoy dutta - 5 years, 9 months ago

there´s no enough info!!!

Reynaldo C. Del Castillo - 5 years, 6 months ago

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Actually there is. Simply assume A is greater, then draw conclusions. Then start over, assume B is greater, and draw conclusions. You will get the same result both times.

Whitney Clark - 5 years, 6 months ago

if and and B both are negative than this will be wrong

Naimish Khara - 7 years, 3 months ago

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It is always correct that first is correct.

Muhammad Abdullah - 5 years, 9 months ago

Why is that? Can you give an example?

Whitney Clark - 5 years, 7 months ago

Yes. If A and B both are negative then it the condition fails..

Srinivas Nani - 5 years, 7 months ago
Petko Petkov
Feb 5, 2014

A A B B A B = A B A B = 1 \frac{A}{A-B} - \frac{B}{A-B} = \frac{A-B}{A-B} = 1 thus A A B > B A B \frac{A}{A-B} > \frac{B}{A-B}

How does it even qualify for a level 5 question it should be a level 1 question dont you agree?

sayan maiti - 7 years, 4 months ago

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Haha I am surprised by the rating it is not expected at all

Mohamed Aly - 7 years, 3 months ago

A=1, B=0. A/(A-B) = 1, B/(A-B)=0 ?

Ng Donn - 7 years, 4 months ago

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Yes, 1>0 which confirms the result.

Petko Petkov - 7 years, 4 months ago

Remember, we are not comparing A and B

Marcus Vinicius M L da Cruz - 5 years, 10 months ago

I just took any random values of both A and B and got tht for any value of A...e.i if u take A is greater than B or A is less than B...in every case u will get first to be greater one. Petko petkov - i like it !!

avilash mahapatra - 7 years, 2 months ago

How do you know which one is which? A or B bananas. I didn't know if A or B was bigger.

Finn Hulse - 7 years, 4 months ago

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Because from the above equality: A A B \frac{A}{A-B} is always bigger than B A B \frac{B}{A-B} as the difference is exactly 1.

Petko Petkov - 7 years, 4 months ago

First assume A=8(any value u can).... then assume B=3... calculate it.... then u assume B=3 and A=8.... still the first one will be greater than the second one...

Arijit Banerjee - 7 years, 4 months ago

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Good point! Oh gosh!

Finn Hulse - 7 years, 4 months ago

i agree with you Sayan maiti

Priyesh Pandey - 7 years, 3 months ago

That's a good one

Kushagra Sahni - 7 years, 3 months ago

wrong proof. You're supponsing a/(a-b) is greater than second

Jose Torres Zapata - 5 years, 11 months ago

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No, he is not. Evaluate B/(A-B) - A/(A-B). Remember, we are not comparing A and B.

Marcus Vinicius M L da Cruz - 5 years, 10 months ago

If A > B A > B , the denominator is positive, and is the same denominator for both. As A > B A > B we conclude that the first one is greater.

If A < B A < B , the denominator is negative, and the same for both.

Case 1 If A A is positive, B B is forced to be positive and hence, the expressions are negative. Since A > B |A| > |B| , this case claims the first expression as the greater.

Case 2 If A A is 0 0 , the first expression is 0 0 , and as B B is forced to be positive, the second expression is negative. 0 0 > than any negative number.

Case 3 If A A is negative, there are three sub-cases:

Sub-Case a: If B B is positive, the first expression is positive, and the second is negative.

Sub-Case b: If B B is 0 0 , the first expression is positive and the second is 0 0 .

Sub-Case c: If B B is negative, both expressions turn to be positive. Remember that the denominator is always negative because A < B A < B . As A > B |A| > |B| , this claims again the first expression as the greater.

We've evaluated and proved that the first expression is always greater than the other. Sorry if there's some mistake.

If A and B are negative, there is no condition given " A<B"

Srinivas Nani - 5 years, 7 months ago

A/(A-B) = (A-B+B)/(A-B) = (A-B)/(A-B) + B/(A-B) = 1 + B/(A-B) . So the first is always 1 more than the second

I did the same thing.... that is remarkable solution with full amount of analysis and reasoning! Good work @Diego E. Nazario Ojeda :)

Sahba Hasan - 5 years, 11 months ago
Mohamed Aly
Feb 2, 2014

Just subtract First -Second = ( A-B)/(A-B)=1 So First must be greater.

A=1, B=0. A/(A-B) = 1, B/(A-B)=0 ?

Ng Donn - 7 years, 4 months ago

True.. I considered all the cases for A,B.. :D

Snehal Shekatkar - 7 years, 4 months ago

Consider 2 cases SINCE A NOT EQUAL TO B EITHER IT MUST BE GREATER OR SMALLER THAN A

1.A>B: THEN FIRST > SECOND

2.A<B: THEN FIRST > SECOND

Anirudha Nayak - 7 years, 4 months ago

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Consider if A is smaller then the denominator will be negatve so in both cases first is greater

Mohamed Aly - 7 years, 4 months ago

Suppose, first, that A > B A > B . Then 1 A B \frac{1}{A-B} is a positive number. We write

A × 1 A B > B × 1 A B A \times \frac{1}{A-B} > B \times \frac{1}{A-B}

Now suppose B > A B > A . Then 1 A B \frac{1}{A-B} is a negative number and we can write

A × 1 A B > B × 1 A B A \times \frac{1}{A-B} > B \times \frac{1}{A-B}

So, either way, A × 1 A B > B × 1 A B A \times \frac{1}{A-B} > B \times \frac{1}{A-B} .

Fatin Farhan
Feb 8, 2014

A A B B A B = 1 \frac{A}{A-B}-\frac{B}{A-B}=1

Sergio Arroyos
Feb 24, 2014

Given that the first minus the second is equal to 1 then the first is greater A A B B A B = A B A B = 1 \frac{A}{A-B} - \frac{B}{A-B} = \frac{A-B}{A-B}=1

Julio Reyes
Feb 8, 2014

First note that if A B t h e n A B A B = 1 A \neq B \quad then \quad {{A - B} \over {A - B}} = 1

If A > B , then:

A B A B = 1 > 0 A A B B A B > 0 A A B > B A B {{A - B} \over { A - B}} = 1 > 0 \quad \to \quad {A \over {A - B}} - {B \over {A - B}} > 0 \quad \to \quad {A \over {A - B}} > {B \over {A - B}}

Same is true if A < B

Julio Reyes - 7 years, 4 months ago
Jon Trou Ble Ton
Feb 8, 2014

first - second=1>0 ergo first >second

Mukund Shridhar
Feb 8, 2014

subtract the second from the first , you get =1 i.e. (A/(A-B))-(B/(A-B) ) = ( (A-B)/(A-B))= 1 [ given , a <> b, and therefore a-b <>0]

therefore expression 1 is > expression 2

Anand Raj
Feb 8, 2014

Let A>B

Let A = 5 ; B = 4

Obviously 5 > 4

Now let A<B

let A = 4 ; B = 5

Again -4 > -5

In both cases The FIRST one is correct.........

Anand - although you got the right answer you actually only proved it for (A,B) = (5,4) and (A,B)=(4,5) : What about the number of other combinations for A &B - for all problems it is ALWAYS to analyse using algebra or logic - rather than guess a few numbers and then assume that one result always holds.

Tony Flury - 5 years, 8 months ago

If A > B A>B , then A B > 0 A-B>0 , which implies A A B > B A B \frac{A}{A-B}>\frac{B}{A-B} .

If A < B A < B , then A B < 0 A-B<0 , which implies A A B > B A B \frac{A}{A-B}>\frac{B}{A-B} . (when divide by negative number, it'll switch the sign.)

Therefore, A A B > B A B \frac{A}{A-B}>\frac{B}{A-B} , for any A , B ; A B A,B; A\neq B .

Skye Rzym
Jun 3, 2017

For all real numbers A A and B B , it's clear that A 2 + B 2 2 A B A^{2}+B^{2}\geq2AB . Since A A is not equal to B, then we get A 2 + B 2 > 2 A B A^{2}+B^{2}>2AB . Then A 2 + B 2 > 2 A B A^{2}+B^{2}>2AB A 2 A B > A B B 2 A^{2}-AB>AB-B^{2} A ( A B ) > B ( A B ) A(A-B)>B(A-B) Divide both sides with ( A B ) 2 (A-B)^{2} , then we get A A B > B A B \frac{A}{A-B}>\frac{B}{A-B} .

Nisarg Thakkar
May 3, 2016

Here, let's take two possibilities. A>B and B>A

First scenario :

If A>B, hence, A/(A-B) > B/(A-B) [since A-B is positive]

If B>A, Then -B<-A. Which means -B/(B-A)<-A/(B-A) [since B-A is positive]

We see that in both cases, A/(A-B) is greater. This is how i understand the problem. There can be many other ways! Thanks!

Eraraya Muten
Jan 16, 2016

For those who still not understand. Look! A is always greater! Lets assume A-B = x

  1. For example, A is greater than B(i said greater, i dont care if they are negative or positive number, the important thing is A greater than B). Then x will be positive. A/x is then greater than B/x or A/x is LESS NEGATIVE than B/x

  2. Now lets assume B is greater than A. Then x will be negative. B/x will be MORE NEGATIVE than A/x or B/x will be smaller than A/x.

No matter what number you put in, you will always get A/x bigger than B/x.

Notes : remember that we have to find which one is bigger, either A/x or B/x, not A or B!

Alan Chen
Jan 14, 2016

I actually solved the problem a simple way.

Let A=2 B=1

then plug those values in the First and Second stuff, and First was greater than Second, so yah.

A A B = A B + B A B = A B A B + B A B = 1 + B A B 1 s t . \dfrac A{ A - B } =\dfrac {A - B + B}{ A - B } =\dfrac {A - B }{ A - B } +\dfrac B{ A - B } =1 + \dfrac B{ A - B }\\ 1st.

I thought the tricky part would arise when I made A a negative number. But I tried a simple example and it turned out the same as if both numbers were positive. If A is -5 and B is 2, then the first fraction becomes -5/-7, which is 5/7. The second fraction becomes 2/-7, which is a neg number, -2/7. So as usual, the former is higher than the latter.

Woody Superman
Dec 12, 2015

We just don't give a f*ck if A and B are positive or negative because First - Second always = (A-B)/(A-B) = 1. --> First = Second + 1 --> First > Second.

f*ck that b=a is positive.

Am Kemplin - 1 month, 2 weeks ago
Daniel Ellesar
Oct 13, 2015

I think you should specify that a low negative number is considered greater than a high negative number. e.g. -2 > -4

Zakir Dakua
Oct 4, 2015

First - Second = A/(A-B) - B/(A-B) = (A-B)/(A-B) = 1. So first one is always greater by 1 than second.

Bryan Hung
Jul 29, 2015

A A B > B A B A A B B A B > 0 A B A B > 0 A , B Z . A B A B = 1 1 > 0 A A B > B A B \frac { A }{ A-B } >\frac { B }{ A-B } \\ \Rightarrow \frac { A }{ A-B } -\frac { B }{ A-B } >0\\ \Rightarrow \frac { A-B }{ A-B } >0\\ \forall A,B\in { Z }.\frac { A-B }{ A-B } =1\\ \Rightarrow 1>0\\ \therefore \frac { A }{ A-B } >\frac { B }{ A-B } \\

Taylor Shobe
Jul 21, 2015

1st - 2nd = 1

=> 1st = 1 + 2nd

=> Therefore; 1st > 2nd

Maria Ashker
Jul 16, 2015

if A>B then the first one is the greater , they have the same denominator but the Numerator of the first is greater so the first one is greater..... if A<B then they are both have negative sign and the first one is greater

Rishi Gangadhar
Jul 14, 2015

if you subtract the second from the first, you get +1, which is clearly greater than zero. Thus, the first one is greater than the second one

Ahmed Obaiedallah
Jun 20, 2015

It will always be the first even if B>A, sure the second |quantity| would be bigger but there will be a negative sign which will lower its value

Marlon Diaz
Mar 23, 2015

ahmm I Do Not solve it but I answerd it right hahahaha I just thought that A is greater than B because A is not equal to B ?? Im right or wrong??

Achu Ashok
Mar 22, 2015

Put any value of B we get negative answer but put any value of A we get possitve answer so the FIRST is greater

Assem Nehad
Mar 20, 2015

Lets take (A-B) = x So A= x+B. B= A-x We have either A/x > B/x or the oppisite So if B/x is bigger we have (x+b)/x <B/x

X+B-B/x <0 we have x/x <0 which means 1 is < 0 If A/x is bigger we have (X+B)/x >B/x then we have

X+B-B/x >0 X/X>0 which means 1>0 therefore A/x have to be bigger or A/A-B

Anna Anant
Jan 24, 2015

The first. If B is larger than A, then A-B is negative so both expressions are negative, so the number with the smaller modulus is larger, this number is the number with the smaller numerator (A) so the first expression is larger. If B is smaller than A then A-B is positive so the expressions are both positive, so the number with the larger modulus is larger, this is the number with the larger numerator (A) so the first expression is largest. Hence in all possible cases the first expression is larger.

Nguyen Tra
Jan 20, 2015

A/(A-B) - B/(A-B) = 1 > 0 => A/(A-B)>B/(A-B)

Gamal Sultan
Jan 20, 2015

Since

A/(A - B) - B/(A - B) = 1

Then

A/(A - B) > B/(A - B)

A/A-B < A

by above A<A(A-B)

1<A-B

1+B<A means

A>B

dividing b/s by A-B

A/(A-B)>B/(A-B)

Lalit Pathak
Nov 23, 2014

Take any example e.g; A=5 and B = 4 and compare ,then Reverse the values, and compare both. One of the ways but not the best !!!

Gilang Syahya
Mar 23, 2014

\frac{A}{A-B} - \frac{B}{A-B}= 1 So, the first value is greater

Andy Cooper
Mar 20, 2014

if a = n and b = n+1 then by substitution you get n/-1 for the first and (n+1)/-1 for the second. so the second is more negative therefore smaller!

Narayan Garimella
Mar 18, 2014

A/A-B is a negative number and hence the smaller negative number will be greater than the higher negative number.

Nicholas Yap
Mar 12, 2014

By sheer logic:

If A is bigger, The first be a smaller negative number than the second, so the first is bigger

If B is bigger, The first will be a larger positive value than the second.

Abhi Manyu
Mar 6, 2014

just subtract the two fractions. first minus second gives one while the second minus first gives -1. second - first<0 implies second < first

Jackie Nguyen
Mar 2, 2014

First - Seccond = 1 >0 => First > Second

Ahmed Abdelbasit
Feb 28, 2014

saying that A is not equal to B may refer to " A < B " so .. A-B is a negative value .. and because A < B A<B
then less negative is greater .. so .. the first is greater ..

Sriram Chandra
Feb 27, 2014

An MCQ can always be answered using examples take (A,B)=(1,2) and (A,B)=(2,3)..and we have the answer.

Rachit Mahendra
Feb 24, 2014

Case 1 :If A < B. Therefore, (A - B) < 0 {negative term} So, A/(A - B) > B/(A - B) . {As, 2 < 3, -2 > -3}.

Case 2: If A > B. Therefore, (A - B) > 0. So, A/(A - B) > B/(A - B)

Case 3: A cannot be equal to zero. {Given}

D K
Feb 22, 2014

CASE 1:If A>B , then A/A-B > B/A-B , since both have same denominator but the numerator A is greater.

CASE 2: If B>A , then A-B is negtaive. So A/A-B > B/A-B, since though B/A-B will be greater in magnitude but the negative sign will make it smaller.

Apoorv Pandey
Feb 21, 2014

Let A>B Then it is obvious that A/A-B>B/A-B Let B>A Then -A/A-B<-B/A-B =>A>B Therefore First value will always be greater than the second one

Kalyan Pakala
Feb 13, 2014

if A>B,

A/A-B >0 and A/A-B >B/A-B

If B>A,

A/A-B <0 and again A/A-B>B/A-B (negative fractions)

Hence the first answer!

(A-B) is negative and A>B. So, A/(A-B) >B/(A-B) Like, let A=5 and B=6 so that A-B= -1 and the A/(A-B)=-5 and B/(A-B)= -6 In this case -5>-6

Aritri Chatterjee
Feb 10, 2014

lets assume , A=5 B=6 , -5 > -6 ,[first condition is B>A] again A=6 B=5 , still 6>5 [ second condition being A>B], so first one is always greater.

Gautham G Ajith
Feb 8, 2014

when A>B FIRST>SECOND; WHEN B>A FIRST WILL BE LESS -VE NO..................SO FIRST>SECOND

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