\(A=\left|x+9\right|+\left|6-x\right|+450\)
\(\left|x+9\right|\ge-x-9\)
\(\left|6-x\right|\ge x-6\)
\(\Rightarrow A\ge-x-9+x-6+45\)
\(\Rightarrow A\ge30\)
xét A = 30 khi
\(\hept{\begin{cases}x+9< 0\\6-x< 0\end{cases}\Rightarrow\hept{\begin{cases}x< -9\\x>6\end{cases}\Rightarrow}voli}\)