Maths Olympiad Prep

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Number theory Difficulty 5.6 AIME, harder Find the answer

Find all the positive perfect cubes that are not divisible by 1010 such that the number obtained by erasing the last three digits is also a perfect cube.

A number or a short expression. Spacing and $ signs are ignored.

Solution

To solve the given problem, we need to find all positive perfect cubes that are not divisible by 1010 and have the property that when the last three digits are erased, the resulting number is also a perfect cube.

1. Understanding the Cube Condition: Let n3 n^3 be a perfect cube such that n30(mod1000) n^3 \equiv 0 \pmod{1000} . This means the last three digits of n3 n^3 are zero, making n3 n^3 a multiple of 10001000. However, the problem requires that n3 n^3 should not be divisible by 10; that is, neither should n3 n^3 end in zero nor should it be divisible by 5.

2. Formulation: Let n3=1000k+m n^3 = 1000k + m , where 0m<1000 0 \leq m < 1000 represents the remaining last three digits. Given that erasing the last three digits of n3 n^3 results in a perfect cube, we have k=j3 k = j^3 for some integer j j .

3. Range of Interest: We need to check for values n3<10000 n^3 < 10000 since we are initially interested in cubes where erasing might yield smaller cubes.

4. Computations:
- For n3=1331 n^3 = 1331, we have n=11 n = 11, since 1331=113 1331 = 11^3. When the last three digits are erased, the number is 1 1, which is 13 1^3.
- For n3=1728 n^3 = 1728, we have n=12 n = 12, since 1728=123 1728 = 12^3. When the last three digits are erased, the number is 1 1, which is 13 1^3.

5. Verification:
- Check if these cubes satisfy the divisibility condition. Both 13311(mod10) 1331 \equiv 1 \pmod{10} and 17288(mod10) 1728 \equiv 8 \pmod{10} are not divisible by 10 10.
- Check if the condition upon erasing the last three digits is a cube. We already verified that in each case the number becomes 11, which is indeed a cube since 1=13 1 = 1^3.

Therefore, the perfect cubes that satisfy the conditions are:
1331 and 1728. \boxed{1331 \text{ and } 1728}.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.