1985 AIME Problems/Problem 13
The numbers in the sequence , , , , are of the form , where For each , let be the greatest common divisor of and . Find the maximum value of as ranges through the positive integers.
If denotes the greatest common divisor of and , then we have . Now assuming that divides , it must divide if it is going to divide the entire expression .
Thus the equation turns into . Now note that since is odd for integral , we can multiply the left integer, , by a power of two without affecting the greatest common divisor. Since the term is quite restrictive, let's multiply by so that we can get a in there.
So . It simplified the way we wanted it to! Now using similar techniques we can write . Thus must divide for every single . This means the largest possible value for is , and we see that it can be achieved when .
We know that and . Since we want to find the GCD of and , we can use the Euclidean algorithm:
Now, the question is to find the GCD of and . We subtract 100 times from . This leaves us with . We want this to equal 0, so solving for gives us . The last remainder is 0, thus is our GCD.
If Solution 2 is not entirely obvious, our answer is the max possible range of . Using the Euclidean Algorithm on and yields that they are relatively prime. Thus, the only way the GCD will not be 1 is if the term share factors with the . Using the Euclidean Algorithm, . Thus, the max GCD is 401.
We can just plug in Euclidean algorithm, to go from to to to get . Now we know that no matter what, is relatively prime to . Therefore the equation can be simplified to: . Subtracting from results in . The greatest possible value of this is , an happens when .
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