Olympiad Maths Prep

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Algebra Difficulty 5.1 AIME, harder Prove it Ukraine

Are there any 10 numbers, not all of which are the same, each of which is equal to the square of the sum of all the other numbers?

Solution

Suppose that such numbers exist. Since they are equal to some squares, each of these numbers is nonnegative. Let's denote the sum of all these 10 numbers by SS. Let's pick one of these numbers and denote it by aa, then SaS \ge a, and also
a=(Sa)2a2(2S+1)a+S2=0. a = (S - a)^2 \Rightarrow a^2 - (2S + 1)a + S^2 = 0.
If there are two different aa, satisfying this condition, then by Vieta's theorem their product is equal to S2S^2. However, this means that one of them is greater than SS, which contradicts what we proved above. Thus, there is only one possible aa, and it follows that all numbers are equal.

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