Root Bombs!

Algebra Level 3

It is given that a a , b b , and c c are non-negative integers satisfying the equation:

a + b + c = a b + b c + c a = 3 a b c a+b+c=ab+bc+ca=3abc

Find the sum of all the possible values of a b c abc .


The answer is 1.

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4 solutions

Let a + b + c = a b + b c + c a = 3 a b c = p a+b+c=ab+bc+ca=3abc=p . Then a , b , c a, b, c are the roots of the equation x 3 p x 2 + p x p 3 = 0 x^3-px^2+px-\dfrac{p}{3}=0 . This equation will have real roots if 27 × p 2 9 + 18 × ( p ) × p × ( p 3 ) 4 p 3 4 × ( p 3 ) × ( p 3 ) + p 2 × p 2 0 -27\times {\dfrac{p^2}{9}}+18\times (-p) \times p\times (-\dfrac{p}{3})-4p^3-4\times (-p^3)\times (-\dfrac{p}{3})+p^2\times p^2\geq 0 . This yields ( p 3 ) 2 0 (p-3)^2\leq 0 . Since ( p 3 ) 2 (p-3)^2 can't be negative, therefore p 3 = 0 p-3=0 or p = 3 p=3 . So 3 a b c = 3 3abc=3 or a b c = 1 abc=\boxed 1

That's a very nice solution. Slight typo/omission - I think you've divided the discriminant through by p p without considering the case p = 0 p=0 (which gives a valid solution, but not a very interesting one!).

Chris Lewis - 1 year, 3 months ago
Chris Lewis
Feb 28, 2020

Without loss of generality, let a b c a \leq b \leq c .

If a = 0 a=0 , we have b + c = b c = 0 b+c=bc=0 ; so b = c = 0 b=c=0 as well.

If a = 1 a=1 , we have 1 + b + c = b + c + b c 1+b+c=b+c+bc , so that b c = 1 bc=1 and b = c = 1 b=c=1 ; checking the remaining equation, we see that this is indeed a solution.

Now assume a > 1 a>1 . Note that a b c a b c ab \leq ca \leq bc , so a b + b c + c a 3 b c < 3 a b c ab+bc+ca \leq 3bc < 3abc ; hence no other solutions exist.

Hence the total of all valid a b c abc is 1 \boxed1 .

Hi. I have got an alternative solution. However I couldn't post it as a solution as some glitch happened and it said incorrect when I typed answer is 1. Anyway here it is. We are given that a, b, c are non negative integers satisfying the equations. a+b+c=ab+bc+ca=3abc. So dividing the second and third parts by abc, we get 1/a+1/b+1/c=3. If a,b, c are positive integers, 1/a, 1/b, 1/c are less than or equal to 1. Equality arises only when a,b,c=1. Hence a,b,c are all 1 as their sum is strictly less than 3 otherwise. Hence, abc=1. So the final answer is 1

Nitin Kumar - 1 year, 3 months ago

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x=y=z=0 will not yield the maximum value of abc

Nitin Kumar - 1 year, 3 months ago

Sorry a b and c

Nitin Kumar - 1 year, 3 months ago

best solution posted by nitin kumar!!

Mohammed Imran - 1 year, 3 months ago
Kushal Dey
Jan 9, 2021

We can write 3abc(a+b+c)=(ab+bc+ca)². Now let bc=x,ca=y,ab=z, our equation transforms to (x+y+z)²=3(xy+yz+zx)=>x²+y²+z²-xy-yz-zx=0. Clearly, x=y=z=>a=b=c.

Tom Engelsman
Feb 29, 2020

Let us start by solving for c c in terms of a a and b b :

a + b 3 a b 1 = c = a b 3 a b ( a + b ) ( 3 b 3 b 2 1 ) a 2 + ( 3 b 2 b ) a b 2 = 0 \frac{a+b}{3ab-1} = c = \frac{ab}{3ab - (a+b)} \Rightarrow (3b - 3b^2 - 1)a^2 + (3b^2 - b)a - b^2 = 0 .

Solving for a a via the Quadratic Formula gives:

a = ( b 3 b 2 ) ± ( 3 b 2 b ) 2 4 ( 3 b 3 b 2 1 ) ( b 2 ) 2 ( 3 b 3 b 2 1 ) = ( b 3 b 2 ) ± 3 b 4 + 6 b 3 3 b 2 2 ( 3 b 3 b 2 1 ) = ( b 3 b 2 ) ± 3 b 2 ( b 1 ) 2 2 ( 3 b 3 b 2 1 ) a = \frac{(b-3b^2) \pm \sqrt{(3b^2 - b)^2 - 4(3b-3b^2-1)(-b^2)}}{2(3b - 3b^2 -1)} = \frac{(b-3b^2) \pm \sqrt{-3b^4 + 6b^3 - 3b^2}}{2(3b - 3b^2 -1)} = \frac{(b-3b^2) \pm \sqrt{-3b^2(b-1)^2}}{2(3b - 3b^2 -1)} (i).

The only way (i) will be real-valued is for b = 0 , 1 b = 0, 1 , which in turn yield a = 0 , 1 a = 0, 1 and c = 0 , 1 c = 0, 1 respectively. These ultimately produce the two products:

a b c = 0 3 abc = 0^3 or a b c = 1 3 0 + 1 = 1 . abc = 1^3 \Rightarrow 0 + 1 = \boxed{1}.

Why will (i) be real valued only for b=0,1?

Mohammed Imran - 1 year, 3 months ago

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Because the above discriminant expression is strictly non-positive for all real b. If b does not equal 0 or 1 (in order to zero-out the negative square-root), then the value of a will end up as a complex number. I would suggest stating 'Non-zero, real integers' if the problem statement needs additional clarification.

tom engelsman - 1 year, 3 months ago

A valid proof is necessary

Mohammed Imran - 1 year, 3 months ago

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I disagree with your comment. The algebra above is soundly presented and provides an alternative solution to the previous ones. Please feel free to provide a proof-based solution of your own if you'd like.

tom engelsman - 1 year, 3 months ago

Yes.That was the proof i was expecting from you!!!

Mohammed Imran - 1 year, 3 months ago

Now, since you have proved it this solution is the simplest solution!!!

Mohammed Imran - 1 year, 3 months ago

Congrats!!!

Mohammed Imran - 1 year, 3 months ago

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