Cho $a,\,b,\,c>0$. Chứng minh:
$$\frac{a^{2}}{\left ( a+ b \right )\left ( 2\,a+ b \right )}+ \frac{b^{2}}{\left ( b+ c \right )\left ( 2\,b+ c \right )}+ \frac{c^{2}}{\left ( c+ a \right )\left ( 2\,c+ a \right )}\geqq \frac{1}{2}$$
Cho $a,\,b,\,c>0$. Chứng minh:
$$\frac{a^{2}}{\left ( a+ b \right )\left ( 2\,a+ b \right )}+ \frac{b^{2}}{\left ( b+ c \right )\left ( 2\,b+ c \right )}+ \frac{c^{2}}{\left ( c+ a \right )\left ( 2\,c+ a \right )}\geqq \frac{1}{2}$$
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