\(P=\sum\frac{a}{\sqrt{a\left(a+b+c\right)+bc}}=\sum\frac{a}{\sqrt{a^2+ab+ac+bc}}=\sum\frac{a}{\sqrt{\left(a+b\right)\left(a+c\right)}}\)
\(P\le\frac{1}{2}\sum\left(\frac{a}{a+b}+\frac{a}{a+c}\right)=\frac{3}{2}\)
\(\Rightarrow P_{max}=\frac{3}{2}\) khi \(a=b=c=\frac{1}{3}\)