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Number theory Difficulty 5.2 AIME, harder Find the answer

Solve the equation a3+b3+c3=2001a^3+b^3+c^3=2001 in positive integers.

Mircea Becheanu, Romania

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

To solve the equation a3+b3+c3=2001 a^3 + b^3 + c^3 = 2001 in positive integers (a,b,c) (a, b, c) , we can explore possible values for a a , b b , and c c that satisfy this equation.

First, calculate some bounds for the integers. Assume abc a \leq b \leq c , which implies 3a32001 3a^3 \le 2001 . Thus, we find:
a320013667a66738.7. a^3 \le \frac{2001}{3} \approx 667 \quad \Rightarrow \quad a \le \sqrt[3]{667} \approx 8.7.
Since a a must be an integer, we have a8 a \le 8 .

Next, try possible values for a a and find corresponding pairs (b,c)(b, c) such that the sum of cubes equals 2001.

### Case a=1 a = 1 :
b3+c3=2000 b^3 + c^3 = 2000
Explore values of b b :
1. b=10 b = 10 , then c3=2000103=20001000=1000c=10 c^3 = 2000 - 10^3 = 2000 - 1000 = 1000 \Rightarrow c = 10 .

Hence, one solution is (a,b,c)=(1,10,10) (a, b, c) = (1, 10, 10) .

### Case a=10 a = 10 :
b3+c3=2001103=20011000=1001 b^3 + c^3 = 2001 - 10^3 = 2001 - 1000 = 1001
Explore values of b b :
1. b=10 b = 10 , then c3=1001103=10011000=1c=1 c^3 = 1001 - 10^3 = 1001 - 1000 = 1 \Rightarrow c = 1 .

Thus, another solution is (a,b,c)=(10,10,1) (a, b, c) = (10, 10, 1) .

### Symmetric Cases:
The nature of the problem allows permutation of a a , b b , and c c , so we get additional solutions by permuting these variables:
- (a,b,c)=(10,1,10) (a, b, c) = (10, 1, 10) .

Therefore, the solutions in positive integers are:
(a,b,c)=(10,10,1),(10,1,10),(1,10,10) \boxed{(a, b, c) = (10, 10, 1), (10, 1, 10), (1, 10, 10)}

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