cho `a,b,c>0`. cmr: `a^2/(a^2+ab+b^2)+b^2/(b^2+bc+c^2)+c^2/(c^2+ca+a^2)>=1`

cho `a,b,c>0`. cmr: `a^2/(a^2+ab+b^2)+b^2/(b^2+bc+c^2)+c^2/(c^2+ca+a^2)>=1`
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