Maths Olympiad Prep

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Algebra Difficulty 4.8 AIME Prove it United States

Problem:

Let aa, bb, and cc be positive real numbers such that
abc=1 a b c = 1
Prove that for every positive integer nn,
a+b+c>1a+1b+1c. a + b + c > \frac{1}{a} + \frac{1}{b} + \frac{1}{c} .

Solution

Solution:

Using abc=1a b c = 1, we transform the given condition to
an+bn+cn>1an+1bn+1cn. a^{n} + b^{n} + c^{n} > \frac{1}{a^{n}} + \frac{1}{b^{n}} + \frac{1}{c^{n}} .
a+b+c>bc+ca+ab a + b + c > b c + c a + a b
or
bccaab+a+b+c>0. - b c - c a - a b + a + b + c > 0 .
We then add abc1(=0)a b c - 1 (= 0) to the left side, getting
abcbccaab+a+b+c1>0 a b c - b c - c a - a b + a + b + c - 1 > 0
which factors as
(a1)(b1)(c1)>0. (a - 1)(b - 1)(c - 1) > 0 .
In exactly the same way we transform the condition to be proved to
(an1)(bn1)(cn1)>0. \left(a^{n} - 1\right)\left(b^{n} - 1\right)\left(c^{n} - 1\right) > 0 .
However, for any positive real xx, the numbers x1x - 1 and xn1x^{n} - 1 are both positive, both negative, or both zero. Consequently (1) and (2) are equivalent.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.