AlgebraDifficulty 5.3AIME, harderProve itUnited States
Problem:
Find all ordered triples (a,b,c) of positive reals that satisfy: ⌊a⌋bc=3, a⌊b⌋c=4, and ab⌊c⌋=5, where ⌊x⌋ denotes the greatest integer less than or equal to x.
Solution
Solution:
Answer: (330,430,5230),(330,230,530)
Write p=abc, q=⌊a⌋⌊b⌋⌊c⌋. Note that q is an integer.
Multiplying the three equations gives: p=q60 Substitution into the first equation, p=3⌊a⌋a<3⌊a⌋⌊a⌋+1≤6 Looking at the last equation: p=5⌊c⌋c≥5⌊c⌋⌊c⌋≥5 Here we've used ⌊x⌋≤x<⌊x⌋+1, and also the apparent fact that ⌊a⌋≥1. Now: 5≤q60≤6512≥q≥35 Since q is an integer, we must have q=2. Since q is a product of 3 positive integers, we must have those be 1, 1, and 2 in some order, so there are three cases:
Case 1: ⌊a⌋=2. By the equations, we'd need a=3230=120/9>3, a contradiction, so there are no solutions in this case.
Case 2: ⌊b⌋=2. We have the solution (330,230,530)
Case 3: ⌊c⌋=2. We have the solution (330,430,5230)
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