Áp cmn dụng BĐT Cauchy Schwarz ta có$\left ( a+b+c \right )\left[ {\frac{a}{\left ( b+c \right )^2}+\frac{b}{\left ( c+a \right )^2}+\frac{c}{\left ( a+b \right )^2}} \right]\geq \left ( \frac{a}{b+c} +\frac{b}{c+a}+\frac{c}{a+b}\right )^2\geq \left ( \frac{3}{2} \right )^2$