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sum a^2/(b + c) =? if sum a/(b + c)=1

Source: Switzerland - 2015 Swiss MO Final Round p7

January 14, 2023
algebraSum

Problem Statement

Let a,b,ca, b, c be real numbers such that: ab+c+bc+a+ca+b=1\frac{a}{b + c}+\frac{b}{c + a}+\frac{c}{a + b}= 1 Determine all values ​​which the following expression can take : a2b+c+b2c+a+c2a+b.\frac{a^2}{b + c} + \frac{b^2}{c + a} + \frac{c^2}{a + b}.