Too much.

{ a b c + 1 b c a + 1 c a b + 1 \left\{ \begin{aligned} a \ &| \ bc + 1\\ b \ &| \ ca + 1\\ c \ &| \ ab + 1\\ \end{aligned} \right. c b a c \ge b \ge a are three positive integers that satisfy the above system of equation and c a + b a b c \ne a + b \ne ab .

Calculate the value of c c .


The answer is 7.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Kazem Sepehrinia
Sep 5, 2017

Multiply three relations to get a b c a b c ( a b c + a + b + c ) + a b + a c + b c + 1 abc \mid abc(abc+a+b+c)+ab+ac+bc+1 . Thus a b c a b + a c + b c + 1 abc \mid ab+ac+bc+1 and the expression 1 a + 1 b + 1 c + 1 a b c \frac{1}{a}+\frac{1}{b}+\frac{1}{c}+\frac{1}{abc} must be a positive integer. If a 4 a \ge 4 , then 1 a + 1 b + 1 c + 1 a b c 3 4 + 1 64 < 1 \frac{1}{a}+\frac{1}{b}+\frac{1}{c}+\frac{1}{abc} \le \frac{3}{4}+\frac{1}{64}<1 Thus a 3 a \le 3 . For a = 1 a=1 , using the same reasoning for bounding a a , one can conclude that b 2 b \le 2 . Similarly for a = 2 a=2 we get 2 b 4 2 \le b \le 4 and for a = 3 a=3 we get b = 3 b=3 .

For ( a , b ) = ( 1 , 1 ) (a, b)=(1, 1) we get 1 a + 1 b + 1 c + 1 a b c = 2 + 2 c \frac{1}{a}+\frac{1}{b}+\frac{1}{c}+\frac{1}{abc}=2+\frac{2}{c} . It gives two solutions ( a , b , c ) = ( 1 , 1 , 1 ) , ( 1 , 1 , 2 ) (a, b, c)=(1, 1, 1), (1, 1, 2) . The first one negates c a b c \neq ab and the second negates c a + b c \neq a+b .

If ( a , b ) = ( 1 , 2 ) (a, b)=(1, 2) we get 1 a + 1 b + 1 c + 1 a b c = 1 + c + 3 2 c \frac{1}{a}+\frac{1}{b}+\frac{1}{c}+\frac{1}{abc}=1+\frac{c+3}{2c} . Thus c = 1 c=1 or c = 2 c=2 . Hence 2 c 2 ( c + 3 ) 2 c = 6 2c \mid 2(c+3)-2c=6 and c 3 c \mid 3 . Thus c = 3 c=3 . It gives the solution ( a , b , c ) = ( 1 , 2 , 3 ) (a, b, c)=(1, 2, 3) . This solution negates the condition c a + b c \neq a+b .

For ( a , b ) = ( 2 , 2 ) (a, b)=(2, 2) we get 1 a + 1 b + 1 c + 1 a b c = 1 + 5 4 c \frac{1}{a}+\frac{1}{b}+\frac{1}{c}+\frac{1}{abc}=1+\frac{5}{4c} , which is never an integer.

If ( a , b ) = ( 2 , 3 ) (a, b)=(2, 3) we get 1 a + 1 b + 1 c + 1 a b c = 1 2 + 1 3 + 1 c + 1 6 c = 5 c + 7 6 c \frac{1}{a}+\frac{1}{b}+\frac{1}{c}+\frac{1}{abc}=\frac{1}{2}+\frac{1}{3}+\frac{1}{c}+\frac{1}{6c}=\frac{5c+7}{6c} . Hence 6 c 6 ( 5 c + 7 ) 5 ( 6 c ) = 42 6c \mid 6(5c+7)-5(6c)=42 and c 7 c \mid 7 . Thus c = 7 c=7 . It gives the solution ( a , b , c ) = ( 2 , 3 , 7 ) (a, b, c)=(2, 3, {\color{#D61F06}7}) .

For ( a , b ) = ( 2 , 4 ) (a, b)=(2, 4) we get 1 a + 1 b + 1 c + 1 a b c = 1 2 + 1 4 + 1 c + 1 8 c = 6 c + 9 8 c \frac{1}{a}+\frac{1}{b}+\frac{1}{c}+\frac{1}{abc}=\frac{1}{2}+\frac{1}{4}+\frac{1}{c}+\frac{1}{8c}=\frac{6c+9}{8c} , which is an odd number divided by an even number and never an integer.

Finally for ( a , b ) = ( 3 , 3 ) (a, b)=(3, 3) we get 1 a + 1 b + 1 c + 1 a b c = 1 3 + 1 3 + 1 c + 1 9 c = 6 c + 10 9 c \frac{1}{a}+\frac{1}{b}+\frac{1}{c}+\frac{1}{abc}=\frac{1}{3}+\frac{1}{3}+\frac{1}{c}+\frac{1}{9c}=\frac{6c+10}{9c} , which is never an integer since numerator is not divisible by 3 3 .

Very nicely done! The first step paves the way for shortening the possibilities, and the reasoning shown in second step makes it easier to brute it out.

Siva Bathula - 3 years, 9 months ago

Log in to reply

Thanks Siva :)

Kazem Sepehrinia - 3 years, 9 months ago

a similar problem was asked in imo

Srikanth Tupurani - 2 years, 3 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...