Mistakes give rise to Problems

Algebra Level 4

We know that the denominator can be distributed in a fraction like a + b c = a c + b c \dfrac{a+b}{c} = \dfrac{a}{c} + \dfrac{b}{c}


But you can't do this in a b + c = a b + a c \dfrac{a}{b+c} = \dfrac{a}{b} + \dfrac{a}{c} It will be a mistake if you do it.

But for how many triples ( a , b , c ) (a,b,c) of positive integers is the above (second equation) property true?


The answer is 0.

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5 solutions

Aditya Raut
Jul 21, 2014

Because the triples are of positive integers, we can cancel out a a from the numerator, because a 0 a\neq 0 as a a is asked to be POSITIVE INTEGER.

Thus we get 1 b + c = 1 b + 1 c \dfrac{1}{b+c} = \dfrac{1}{b} + \dfrac{1}{c} .

But because b b and c c are also positive integers, we can say that b + c > b 1 b + c < 1 b b+c > b \implies \dfrac{1}{b+c} < \dfrac{1}{b} a n d and b + c > c 1 b + c < 1 c b+c>c \implies \dfrac{1}{b+c} < \dfrac{1}{c}

And this results in 1 b + c < 1 b + 1 c . . . . . . . . . . . b , c N \dfrac{1}{b+c} < \dfrac{1}{b} + \dfrac{1}{c}........... \forall b,c \in \mathbb{N}

Thus the required statement is not true for any triples ( a , b , c ) (a,b,c) , hence the answer is 0 \boxed{0}

I did it in this way (considering a,b,c as non-zero reals).

Lets assume that for a triple ( a , b , c ) (a,b,c) , the equation a b + c = a b + a c \frac{a}{b+c}=\frac{a}{b}+\frac{a}{c} holds true.

After a little calculation and cancelling out a a from both sides since it's non-zero, we get,

b 2 + c 2 + b c = 0 b^2+c^2+bc=0

Now, since b 2 b^2 and c 2 c^2 are always positive, we should have b c = 0 bc=0 and thus the solution would be b = 0 b=0 or c = 0 c=0 or b = 0 c = 0 b=0 \land c=0 .

But a , b , c a,b,c are non-zero reals. So, our original assumption was wrong. Thus, there exists no triple ( a , b , c ) , a , b , c R { 0 } (a,b,c), a,b,c \in R-\{0\} that satisfies the given equation.

Prasun Biswas - 6 years, 9 months ago

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Same solution. For the equation to be true, either b b or c c must be negative.

Roman Frago - 5 years, 10 months ago

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Not quite. There exists no triple ( a , b , c ) (a,b,c) of non-zero reals a , b , c a,b,c that satisfy the given equation. So, even for negative values of b , c b,c , there doesn't exist any non-zero real a a satisfying the equation.

Prasun Biswas - 5 years, 10 months ago

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@Prasun Biswas Since both b 2 b^2 and c 2 c^2 are positive, how can b 2 + c 2 + b c b^2 + c^2 + bc be equal to 0 0 ? Only by either b = c = 0 b=c=0 or b = ( 1 ± 3 i 2 ) × c b=-(\frac {1 \pm \sqrt 3i}{2})\times c .

Roman Frago - 5 years, 10 months ago

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@Roman Frago You meant that kind of negative? I thought you meant negative real. You should've said that you're considering non-real numbers then. My method explicitly assumed a , b , c a,b,c as non-zero reals (hence I didn't think about non-real complex values). Sorry for the misunderstanding.

Prasun Biswas - 5 years, 10 months ago

If you're given that a a is non-zero, then there can be no ordered triple [they dont even have to be integers] even if you allow b b and c c to be negative.

Mursalin Habib - 6 years, 10 months ago

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But i wanted to give the condition only on a a (when it gets positive it's non-zero) but to make it look better, i gave to all ...

Aditya Raut - 6 years, 10 months ago

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First, note that we can divide both sides by a a . Now, if there existed a solution to b , c b,c , then there would be an infinite number of solutions, since we can take a a to be any positive integer. Thus there are no solutions.

Daniel Liu - 6 years, 10 months ago

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@Daniel Liu I also did the same

Aditya Chauhan - 5 years, 11 months ago

Correct. No real number would satisfy the equation if a 0 a\neq0 .

Roman Frago - 5 years, 10 months ago

Wow... that's a cool set you've shared.

Led Tasso - 6 years, 10 months ago

Working the expression and you'll get that the only possible solution could be when a ( c 2 + b c + b 2 ) = 0 a(c^2 + bc + b^2) = 0 But a can't be 0, because the triples may only be positive integers. Note that c 2 + b c + b 2 = 0 c^2 + bc + b^2 = 0 if both b and c ar equal to 0, that's impossible because c can't be replaced by 0 on the first expression

Thus, there are no triples of positive integers such that the statement is true

bc=(b+c)^2 b^2+bc+c^2=0 This cannot be '0' for any positive values of b and c

so the no.of triplets = 0

Rama Krishna Paramahamsa - 6 years, 10 months ago

oh.......... I see, thanks!!!

Kenny Santoso - 6 years, 10 months ago

Since a a , b b & c c are all positive integers : a b + c = a b + a c \dfrac{a}{b+c} = \dfrac{a}{b} + \dfrac{a}{c}

By multiplying both sides by 1 a \frac{1}{a} , we get : 1 b + c = 1 b + 1 c \dfrac{1}{b+c} = \dfrac{1}{b} + \dfrac{1}{c} b + c b + b + c c = 1 = b b + c b + b c + c c = c b + b c + 2 \therefore \ \ \dfrac{b+c}{b} + \dfrac{b+c}{c} = 1 = \dfrac{b}{b} + \dfrac{c}{b} + \dfrac{b}{c} + \dfrac{c}{c} = \dfrac{c}{b} + \dfrac{b}{c} +2 c b + b c = 1 \therefore \ \ \dfrac{c}{b} + \dfrac{b}{c} = -1 c 2 b c + b 2 b c = b 2 + c 2 b c = 1 \therefore \ \ \dfrac{c^2}{bc} + \dfrac{b^2}{bc} = \dfrac{b^2 + c^2}{bc} = -1 b 2 + c 2 = b c \therefore \ \ b^2 + c^2 = -bc

But as we see , as both b b and c c are positive integers , Hence b 2 + c 2 b^2 + c^2 is a positive value and also b c -bc is a negative value . Which leads us to a contradiction and an impossible argument .

And thus , there is no triple ( a , b , c ) (a, b, c) (given the above conditions) satisfying the equation . And the answer is 0 \boxed{0} .

Arulx Z
Jul 14, 2015

a b + c = a b + a c 1 b + c = 1 b + 1 c 1 b + c = b + c b c b c = b 2 + 2 b c + c 2 b 2 + b c + c 2 = 0 {\frac { a }{ b+c } =\frac { a }{ b } +\frac { a }{ c } \\ \frac { 1 }{ b+c } =\frac { 1 }{ b } +\frac { 1 }{ c } \\ \frac { 1 }{ b+c } =\frac { b+c }{ bc } \\ bc={ b }^{ 2 }+2bc+{ c }^{ 2 }\\ { b }^{ 2 }+bc+{ c }^{ 2 }=0}

The equation above cannot be true for any positive integer as square of any positive integer is always positive. Also since both b b and c c are positive, b c bc will always be positive.

Hence no triplet satisfies the equation.

Moderator note:

You should still add a line about why it cannot be true for any positive integer.

Same way!!

Dev Sharma - 5 years, 7 months ago
Vinayak Kumar
Sep 27, 2014

Note that if there was a triple (a,b,c) that worked, then we could replace a with any other positive integer in order to make another working triple. But this would mean the answer is infinitely many and that can't be inputted, so the answer must be 0 :D.

But I actually did it the way it was meant to be in order to make sure i was right.

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