Maths Olympiad Prep

Library / /593 of 860

Algebra Difficulty 5.3 AIME, harder Find the answer

For how many ordered triples (a,b,c)(a, b, c) of positive integers are the equations abc+9=ab+bc+caabc+9=ab+bc+ca and a+b+c=10a+b+c=10 satisfied?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Subtracting the first equation from the second, we obtain 1abc+ab+bc+caabc=(1a)(1b)(1c)=01-a-b-c+ab+bc+ca-abc=(1-a)(1-b)(1-c)=0. Since a,ba, b, and cc are positive integers, at least one must equal 1. Note that a=b=c=1a=b=c=1 is not a valid triple, so it suffices to consider the cases where exactly two or one of a,b,ca, b, c are equal to 1. If a=b=1a=b=1, we obtain c=8c=8 and similarly for the other two cases, so this gives 3 ordered triples. If a=1a=1, then we need b+c=9b+c=9, which has 6 solutions for b,c1b, c \neq 1; a similar argument for bb and cc gives a total of 18 such solutions. It is easy to check that all the solutions we found are actually solutions to the original equations. Adding, we find 18+3=2118+3=21 total triples.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.