Cho a,b,c>0. Tìm min:
P=$\frac{b^{3}}{a\left ( b^{2}+c^{2} \right )}+\frac{ca^{2}}{b\left ( a^{2}+b^{2} \right )}+\frac{5ac}{\left ( a+c \right )^{2}}-\frac{6\sqrt{ac}}{a+c}$