⎩ ⎪ ⎨ ⎪ ⎧ a b c ∣ b c + 1 ∣ c a + 1 ∣ a b + 1 c ≥ b ≥ a are three positive integers that satisfy the above system of equation and c = a + b = a b .
Calculate the value of c .
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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.
a similar problem was asked in imo
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Multiply three relations to get a b c ∣ a b c ( a b c + a + b + c ) + a b + a c + b c + 1 . Thus a b c ∣ a b + a c + b c + 1 and the expression a 1 + b 1 + c 1 + a b c 1 must be a positive integer. If a ≥ 4 , then a 1 + b 1 + c 1 + a b c 1 ≤ 4 3 + 6 4 1 < 1 Thus a ≤ 3 . For a = 1 , using the same reasoning for bounding a , one can conclude that b ≤ 2 . Similarly for a = 2 we get 2 ≤ b ≤ 4 and for a = 3 we get b = 3 .
For ( a , b ) = ( 1 , 1 ) we get a 1 + b 1 + c 1 + a b c 1 = 2 + c 2 . It gives two solutions ( a , b , c ) = ( 1 , 1 , 1 ) , ( 1 , 1 , 2 ) . The first one negates c = a b and the second negates c = a + b .
If ( a , b ) = ( 1 , 2 ) we get a 1 + b 1 + c 1 + a b c 1 = 1 + 2 c c + 3 . Thus c = 1 or c = 2 . Hence 2 c ∣ 2 ( c + 3 ) − 2 c = 6 and c ∣ 3 . Thus c = 3 . It gives the solution ( a , b , c ) = ( 1 , 2 , 3 ) . This solution negates the condition c = a + b .
For ( a , b ) = ( 2 , 2 ) we get a 1 + b 1 + c 1 + a b c 1 = 1 + 4 c 5 , which is never an integer.
If ( a , b ) = ( 2 , 3 ) we get a 1 + b 1 + c 1 + a b c 1 = 2 1 + 3 1 + c 1 + 6 c 1 = 6 c 5 c + 7 . Hence 6 c ∣ 6 ( 5 c + 7 ) − 5 ( 6 c ) = 4 2 and c ∣ 7 . Thus c = 7 . It gives the solution ( a , b , c ) = ( 2 , 3 , 7 ) .
For ( a , b ) = ( 2 , 4 ) we get a 1 + b 1 + c 1 + a b c 1 = 2 1 + 4 1 + c 1 + 8 c 1 = 8 c 6 c + 9 , which is an odd number divided by an even number and never an integer.
Finally for ( a , b ) = ( 3 , 3 ) we get a 1 + b 1 + c 1 + a b c 1 = 3 1 + 3 1 + c 1 + 9 c 1 = 9 c 6 c + 1 0 , which is never an integer since numerator is not divisible by 3 .