MathDB
Estonian Math Competitions 2005/2006

Source: Seniors Problem 4

July 30, 2008
algorithmgreatest common divisorcombinatorics unsolvedcombinatorics

Problem Statement

Martin invented the following algorithm. Let two irreducible fractions s1t1 \frac{s_1}{t_1} and s2t2 \frac{s_2}{t_2} be given as inputs, with the numerators and denominators being positive integers. Divide s1 s_1 and s2 s_2 by their greatest common divisor c c and obtain a1 a_1 and a2 a_2, respectively. Similarily, divide t1 t_1 and t2 t_2 by their greatest common divisor d d and obtain b1 b_1 and b2 b_2, respectively. After that, form a new fraction \frac{a_1b_2 \plus{} a_2b_1}{t_1b_2}, reduce it, and multiply the numerator of the result by c c. Martin claims that this algorithm always finds the sum of the original fractions as an irreducible fraction. Is his claim correct?