2022 AMC 10A Problems/Problem 19
Define as the least common multiple of all the integers from to inclusive. There is a unique integer such that What is the remainder when is divided by ?
Notice that contains the highest power of every prime below since higher primes cannot divide . Thus, .
When writing the sum under a common fraction, we multiply the denominators by divided by each denominator. However, since is a multiple of , all terms will be a multiple of until we divide out , and the only term that will do this is . Thus, the remainder of all other terms when divided by will be , so the problem is essentially asking us what the remainder of divided by is. This is equivalent to finding the remainder of divided by .
We use modular arithmetic to simplify our answer:
This is congruent to .
Evaluating, we get: Therefore the remainder is .
As in solution 1, we express the LHS as a sum under one common denominator. We note that
Now, we have . We'd like to find so we can evaluate our expression Since don't have a factor of in their denominators, and since is a multiple of multiplying each of those terms and adding them will get a multiple of , that result is Thus, we only need to consider Proceed with solution to get .
Using Wolstenholmes' Theorem, we can rewrite as (for some ). Adding the to , we get .
Now we have and we want . We find that . Taking and multiplying, we get .
Applying Wilson's Theorem on and reducing, we simplify the congruence to . Now we proceed with Solution 1 and find that , so our answer is .
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