dkxd: a> hoặc =0, khác 9
\(a,P=\dfrac{81-9}{9-3}=\dfrac{72}{6}=12\\ b,Q=\dfrac{3\sqrt{a}+9+2\sqrt{a}-6+a-5\sqrt{a}-3}{\left(\sqrt{a}-3\right)\left(\sqrt{a}+3\right)}\\ Q=\dfrac{a}{\left(\sqrt{a}-3\right)\left(\sqrt{a}+3\right)}\\ c,A=PQ=\dfrac{a\left(\sqrt{a}-3\right)\left(\sqrt{a}+3\right)}{\left(\sqrt{a}-3\right)^2\left(\sqrt{a}+3\right)}=\dfrac{a}{\sqrt{a}-3}\\ A=\dfrac{a-9+9}{\sqrt{a}-3}=\sqrt{a}+3+\dfrac{9}{\sqrt{a}-3}\\ A=\sqrt{a}-3+\dfrac{9}{\sqrt{a}-3}+6\ge2\cdot3+6=12\\ A_{min}=12\Leftrightarrow\left(\sqrt{a}-3\right)^2=9\Leftrightarrow\left[{}\begin{matrix}\sqrt{a}-3=3\\\sqrt{a}-3=-3\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}a=36\left(tm\right)\\a=0\left(tm\right)\end{matrix}\right.\)
