\(\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{c+a}=\frac{1}{7}\)

\(\Rightarrow\frac{a+b+c}{a+b}+\frac{a+b+c}{b+c}+\frac{a+b+c}{c+a}=\frac{1}{7}\left(a+b+c\right)\) (nhân a + b +c vào mỗi vế)

\(\Rightarrow3+\frac{a}{b+c}+\frac{b}{c+a}+\frac{c}{a+b}=\frac{2009}{7}\)

Suy ra \(S=\frac{2009}{7}-3=284\)

Ta có :

\(S+3=\left(\frac{a}{b+c}+1\right)+\left(\frac{b}{a+c}+1\right)+\left(\frac{c}{a+b}+1\right)\)

\(=\left(\frac{a}{b+c}+\frac{b+c}{b+c}\right)+\left(\frac{b}{a+c}+\frac{a+c}{a+c}\right)+\left(\frac{c}{a+b}+\frac{a+b}{a+b}\right)\)

\(=\frac{a+b+c}{b+c}+\frac{a+b+c}{a+c}+\frac{a+b+c}{a+b}\)

\(=\left(a+b+c\right)\left(\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{c+a}\right)\)

\(=2009.\frac{1}{7}=287\)

\(\Rightarrow S=287-3=284\)

\(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}=1\)=> \(\frac{bc+ac+ab}{abc}=1\) => bc + ac + ab - abc = 0

<=> c.(a + b) + ab.(1 - c) = 0 

<=> c.(a + b) + ab. (a + b) = 0 <=> (a + b).(c + ab) = 0

<=> (a+ b).(1 - a - b + ab) = 0 <=> (a + b).[(1- b) - a.(1 - b)] = 0 <=> (a + b). (1 - a).(1 - b) = 0 

<=> a + b = hoặc 1 - a = 0 hoặc 1 - b = 0

+) a + b = 0 => a = - b và c = 1 => S = a²⁰⁰⁹ + b²⁰⁰⁹ + c²⁰⁰⁹ = (-b)²⁰⁰⁹ + b²⁰⁰⁹ + 1²⁰⁰⁹ = 1

+) a = 1 => b + c = 0 => b = - c . tương tự => S = 1

+) b = 1. tương tự => S = 1

Vậy S = 1

\(a+b+c=1=\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\)

\(\Rightarrow\frac{1}{a+b+c}=\frac{1}{a}+\frac{1}{b}+\frac{1}{c}=1\)

\(\Rightarrow\frac{1}{a+b+c}=\frac{ab+bc+ca}{abc}\Leftrightarrow\left(a+b+c\right)\left(ab+bc+ca\right)=abc\)

\(\Leftrightarrow\)\(\left(a+b\right)\left(ab+bc+ca\right)+abc+bc^2+c^2a-abc=0\)

\(\Leftrightarrow\left(a+b\right)\left(ab+bc+ca\right)+\left(a+b\right).c^2=0\)

\(\Leftrightarrow\left(a+b\right)\left[ab+bc+ca+c^2\right]=0\)

\(\Leftrightarrow\left(a+b\right)\left(b+c\right)\left(c+a\right)=0\)

\(\Leftrightarrow a+b=0\text{ hoặc }b+c=0\text{ hoặc }c+a=0\)

Do vai trò của a, b, c là như nhau nên không mất tính tổng quát, giả sử b + c = 0.\(b+c=0\Leftrightarrow b=-c\Rightarrow b^{2009}+c^{2009}=\left(-c\right)^{2009}+c^{2009}=-c^{2009}+c^{2009}=0\)

\(1=a+b+c=a+0=a\)

\(\Rightarrow a^{2009}+b^{2009}+c^{2009}=1^{2009}+0=1\text{ (đpcm)}\)