Define a polynomial f ( x ) = ( x a ) ( x b ) ( x c ) f(x) = (x-a)(x-b)(x-c)

a + b + c + a b + b c + a c = a b c + 1 \large a + b + c + ab + bc + ac = abc + 1

Determine the number of triplets of positive integers ( a , b , c ) (a,b,c) with c b a c\geq b \geq a such that the equation above is fulfilled.

This problem appeared in RMO 2005.(#6)


The answer is 3.

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1 solution

Patrick Corn
Feb 26, 2015

Let d = a b c ( a + b + c ) = ( a b + b c + a c ) 1 d = abc-(a+b+c) = (ab+bc+ac)-1 . Note d > 0 d > 0 . Then ( a + i ) ( b + i ) ( c + i ) = d + d i (a+i)(b+i)(c+i) = d+di so the complex argument of the product is 4 5 o 45^o , and the arguments of the terms are between 0 0 and 4 5 o 45^o , and the arguments of the terms are in decreasing order and sum to 4 5 o 45^o , so arg ( a + i ) (a+i) is at least 1 5 o 15^o , so a 1 tan ( 1 5 o ) 3.732 a \le \frac1{\tan(15^o)} \approx 3.732 , so a = 1 , 2 , 3 a =1,2,3 are the only possibilities.

a = 1 a = 1 simplifies to b + c = 0 b+c = 0 , which is impossible.

a = 2 a = 2 simplifies to ( b 3 ) ( c 3 ) = 10 (b-3)(c-3)=10 , which leads to ( 2 , 4 , 13 ) (2,4,13) and ( 2 , 5 , 8 ) (2,5,8) .

a = 3 a = 3 simplifies to ( b 2 ) ( c 2 ) = 5 (b-2)(c-2) = 5 , which leads to ( 3 , 3 , 7 ) (3,3,7) .

So there are a total of 3 \fbox{3} solutions.

There is probably a way of getting the inequality without using complex analysis, but I think this way is pretty clean.

I couldn't understand the first part of the solution. Can you please explain it to me?

Anupam Nayak - 5 years, 6 months ago

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