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A very simple one!!!!

Source: India RMO 1991 Problem 2

October 15, 2005
inequalities

Problem Statement

If a,b,c,da,b,c,d be any four positive real numbers, then prove that ab+bc+cd+da≥4. \frac{a}{b} + \frac{b}{c} + \frac{c}{d} + \frac{d}{a} \geq 4.
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