# AM-GM Inequality

In the case of two variables, the Arithmetic Mean - Geometric Mean (AM-GM) inequality -

Among all rectangles of a given area the square has the least perimeter.

Or, equivalently,

Among all rectangles of a given perimeter the square has the largest area.

This duality of the formulation carries over to the AM-AG inequality.

For positive a, b that satisfy a + b = 2, ab ≤ 1.

Equivalently,

For positive a, b that satisfy ab = 1, a + b ≥ 2.

While trivial, it is often useful while solving problems to keep this interpretation in mind. Here's one example from the 1935 Moscow Mathematical Olympiad:

Find all real solutions of the following system:

x + y = 2

xy - z² = 1.

### Solution

Since x + y = 2, xy ≤ 1 so that

1 = xy - z² ≤ 1 - z² < 1,

unless z = 0. To avoid a contradiction (1 < 1), we have to accept

|Contact| |Front page| |Contents| |Did you know?| |Geometry|

Copyright © 1996-2018 Alexander Bogomolny68559346