Let a, b, and c be positive real numbers such that abc=1 Prove that for every positive integer n, a+b+c>a1+b1+c1.
Solution
Solution:
Using abc=1, we transform the given condition to an+bn+cn>an1+bn1+cn1. a+b+c>bc+ca+ab or −bc−ca−ab+a+b+c>0. We then add abc−1(=0) to the left side, getting abc−bc−ca−ab+a+b+c−1>0 which factors as (a−1)(b−1)(c−1)>0. In exactly the same way we transform the condition to be proved to (an−1)(bn−1)(cn−1)>0. However, for any positive real x, the numbers x−1 and xn−1 are both positive, both negative, or both zero. Consequently (1) and (2) are equivalent.
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Source: MathNet,
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