2009 AIME I Problems/Problem 14
For , define , where . If and , find the minimum possible value for .
Because the order of the 's doesn't matter, we simply need to find the number of s s s and s that minimize . So let and represent the number of s, s, s, and s respectively. Then we can write three equations based on these variables. Since there are a total of s, we know that . We also know that and . We can now solve these down to two variables: Substituting this into the second and third equations, we get and The second of these can be reduced to Now we substitute from the first new equation into the other new equation. Since and are integers, the two solutions to this are or . If you plug both these solutions in to it is apparent that the second one returns a smaller value. It turns out that , , , and , so .
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