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Algebra Difficulty 4.6 AIME Prove it Soviet Union

Problem:
The product of three positive numbers is 11, their sum is greater than the sum of their inverses. Prove that just one of the numbers is greater than 11.

Solution

Solution:
The product of the numbers is 11, so they cannot all be greater than 11 or all less than 11. If all equalled 11, then the sum would not be greater than the sum of the inverses. So we must have either one or two greater than 11. Thus it is sufficient to show that we cannot have two of the numbers greater than 11.

Suppose that aa, b>1b > 1. Then since a+b+c>1/a+1/b+1/ca + b + c > 1/a + 1/b + 1/c, we have a+b+1/(ab)>1/a+1/b+aba + b + 1/(ab) > 1/a + 1/b + ab, and hence (11/a)(11/b)>(a1)(b1)(1 - 1/a)(1 - 1/b) > (a - 1)(b - 1). Dividing by (a1)(b1)(a - 1)(b - 1) gives ab<1ab < 1. Contradiction.

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