Maths Olympiad Prep

Track / Stage 3 / 176 of 260 #176 of 1964

Problem 176

AMC 10/12, early questions
Number theory Difficulty 3.6 Multiple choice

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

Pick one

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Official solution

Prime factorizing 2019201^9, we get 39⋅6793^9\cdot67^9.
A perfect square must have even powers of its prime factors, so our possible choices for our exponents to get perfect square are 0,2,4,6,80, 2, 4, 6, 8 for both 33 and 6767. This yields 5⋅5=255\cdot5 = 25 perfect squares.
Perfect cubes must have multiples of 33 for each of their prime factors' exponents, so we have either 0,3,60, 3, 6, or 99 for both 33 and 6767, which yields 4⋅4=164\cdot4 = 16 perfect cubes, for a total of 25+16=4125+16 = 41.
Subtracting the overcounted powers of 66 (30⋅6703^0\cdot67^0 , 30⋅6763^0\cdot67^6 , 36⋅6703^6\cdot67^0, and 36⋅6763^6\cdot67^6), we get 41−4=(C) 3741-4 = \boxed{\textbf{(C) }37}.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.