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Number theory Difficulty 6.9 National olympiad Prove it

Let mm be the number of ordered solutions (a,b,c,d,e)(a,b,c,d,e) satisfying:
1)1) a,b,c,d,eZ+a,b,c,d,e\in \mathbb{Z}^{+};
2)2) 1a+1b+1c+1d+1e=1\frac{1}{a}+\frac{1}{b}+\frac{1}{c}+\frac{1}{d}+\frac{1}{e}=1;
Prove that mm is odd.

Solution

1. Consider the general form of the equation:
1a+1b+1c+1d+1e=1 \frac{1}{a} + \frac{1}{b} + \frac{1}{c} + \frac{1}{d} + \frac{1}{e} = 1
where a,b,c,d,eZ+a, b, c, d, e \in \mathbb{Z}^{+}.

2. **Count the solutions where aba \neq b:**
If (a,b,c,d,e)(a, b, c, d, e) is a solution, then (b,a,c,d,e)(b, a, c, d, e) is also a solution. These pairs are distinct and thus the number of such solutions is even.

3. **Consider the case where a=ba = b:**
The equation simplifies to:
2a+1c+1d+1e=1 \frac{2}{a} + \frac{1}{c} + \frac{1}{d} + \frac{1}{e} = 1
Similarly, if (c,d,e)(c, d, e) is a solution, then (d,c,e)(d, c, e) is also a solution, making the number of such solutions even.

4. **Consider the case where a=ba = b and c=dc = d:**
The equation further simplifies to:
2a+2c+1e=1 \frac{2}{a} + \frac{2}{c} + \frac{1}{e} = 1
Again, if (a,c,e)(a, c, e) is a solution, then (c,a,e)(c, a, e) is also a solution, making the number of such solutions even.

5. **Consider the case where a=ba = b, c=dc = d, and a=ca = c:**
The equation now becomes:
4a+1e=1 \frac{4}{a} + \frac{1}{e} = 1
Solving for ee, we get:
4a=11e    4ea=e1    4e=aea    a(e1)=4e    (a4)(e1)=4 \frac{4}{a} = 1 - \frac{1}{e} \implies \frac{4e}{a} = e - 1 \implies 4e = ae - a \implies a(e - 1) = 4e \implies (a - 4)(e - 1) = 4

6. **Find the integer solutions to (a4)(e1)=4(a - 4)(e - 1) = 4:**
The integer pairs (a4,e1)(a - 4, e - 1) that satisfy this equation are:
(1,4),(2,2),(4,1) (1, 4), (2, 2), (4, 1)
This gives us the solutions:
(a,e)=(5,5),(6,3),(8,2) (a, e) = (5, 5), (6, 3), (8, 2)

7. Count the total number of solutions:
Since the number of solutions where aba \neq b and other similar cases are even, and we have exactly 3 solutions where a=b=c=da = b = c = d, the total number of solutions is odd.

\blacksquare

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.