Olympiad Maths Prep

Track / Stage 7 / 101 of 300 #1501 of 2000

Problem 1501

National olympiad second round; IMO P1/P4
Number theory Difficulty 7.2 Prove it

We have n>2n>2 non-zero integers such that each one of them is divisible by the sum of the other n1n-1 numbers. Prove that the sum of all the given numbers is zero.

This one wants a proof. Work it on paper, read the official solution, then mark yourself honestly — the ladder only means something if the record is true.

Official solution

1. Let a1,a2,,an a_1, a_2, \dots, a_n be the given non-zero integers, and let S=i=1nai S = \sum_{i=1}^n a_i be the sum of these integers.
2. According to the problem, each ai a_i is divisible by the sum of the other n1 n-1 numbers. This can be written as:
Saiaifor all i. S - a_i \mid a_i \quad \text{for all } i.
3. This implies that SaiS S - a_i \mid S for all i i . Let ki k_i be an integer such that:
ki=SSai. k_i = \frac{S}{S - a_i}.
4. Rearranging the equation, we get:
ai=S(ki1)ki. a_i = \frac{S \cdot (k_i - 1)}{k_i}.
5. Summing up all ai a_i , we have:
S=i=1nai=i=1nS(ki1)ki. S = \sum_{i=1}^n a_i = \sum_{i=1}^n \frac{S \cdot (k_i - 1)}{k_i}.
6. Factoring out S S from the sum, we get:
S=Si=1n(11ki). S = S \cdot \sum_{i=1}^n \left(1 - \frac{1}{k_i}\right).
7. If S0 S \neq 0 , we can divide both sides by S S :
1=i=1n(11ki). 1 = \sum_{i=1}^n \left(1 - \frac{1}{k_i}\right).
8. Simplifying the right-hand side, we get:
1=ni=1n1ki. 1 = n - \sum_{i=1}^n \frac{1}{k_i}.
9. Rearranging the equation, we obtain:
i=1n1ki=n1. \sum_{i=1}^n \frac{1}{k_i} = n - 1.
10. Since ki k_i are integers, the only possible values for ki k_i that satisfy this equation are k1=k2==kn2=1 k_1 = k_2 = \dots = k_{n-2} = 1 and kn1=kn=2 k_{n-1} = k_n = 2 . However, this leads to a contradiction because if ki=1 k_i = 1 , then ai=0 a_i = 0 , which contradicts the given condition that all ai a_i are non-zero.
11. Therefore, the assumption S0 S \neq 0 must be false, implying that S=0 S = 0 .

The final answer is S=0 \boxed{ S = 0 } .

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