(a/b+b/c+c/a)²
≥ (a/b+b/c+c/a)* 4( ab/(b+c)² + bc/(c+a)² + ca/(a+b)²)
≥ 4[a/(b+c)+b/(c+a)+c/(a+c)]²
a/b+b/c+c/a ≥2 [a/(b+c)+b/(c+a)+c/(a+c)]
≥（c+a）/(c+b)+(a+b)/(a+c)+(b+c)/(b+a)