2019 AMC 10A Problem 11

Attempt Problem 11 of the 2019 AMC 10A below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2019 AMC 10A solutions, or check the answer key.

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11.

How many positive integer divisors of 2019201^9 are perfect squares or perfect cubes (or both)?

3232

3636

3737

3939

4141

Answer: C
Concepts:prime factorizationperfect powerinclusion-exclusion
Difficulty rating: 1480
Solution:

Taking the prime factorization of 2019,201^9, we get 39679.3^9 \cdot 67^9.

Note that a perfect square has even exponents for its prime factors, and a cube's exponents are divisible by 3.3.

There are 55 options for an even exponent, from 00 through 8,8, and 44 options for multiples of 3,3, from 00 through 9.9.

This gives us 525^2 options for the squares and 424^2 options for the cubes. We have to subtract out the sixth powers, however.

Using the same logic, sixth powers have to have exponents of prime factors be divisible by 6.6. There are 22 options, 00 and 6.6.

This means that there are 22=42^2 = 4 sixth powers. This gives us a total of 25+164=37. 25 + 16 - 4 = 37. perfect squares or perfect cubes.

Thus, C is the correct answer.

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