2001 AIME II Problems/Problem 5
A set of positive numbers has the triangle property if it has three distinct elements that are the lengths of the sides of a triangle whose area is positive. Consider sets of consecutive positive integers, all of whose ten-element subsets have the triangle property. What is the largest possible value of ?
Out of all ten-element subsets with distinct elements that do not possess the triangle property, we want to find the one with the smallest maximum element. Call this subset . Without loss of generality, consider any with . does not possess the triangle property, so . We use this property to build up from the smallest possible and :
is the "smallest" ten-element subset without the triangle property, and since the set is the largest set of consecutive integers that does not contain this subset, it is also the largest set of consecutive integers in which all ten-element subsets possess the triangle property. Thus, our answer is .
If we wanted to find this for a much larger number (say 2001), we could have noted that this is actually a "quasi-Fibonacci" sequence with initial terms and built up an explicit function to find the term. (The latter part is generally pretty annoying).
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