Ta có: \(\left(y^2-25\right)^6\ge0\forall x\)
\(\Rightarrow100-\left(y^2-25\right)^6\le100\forall x\)
\(\Rightarrow A\le100\)
Dấu \("="\) xảy ra khi \(\left(y^2-25\right)^6=0\)
\(\Rightarrow y^2-25=0\) \(\Rightarrow y^2=5^2\)
\(\Rightarrow y=\left[\begin{matrix}5\\-5\end{matrix}\right.\)
Vậy \(Max_A=100\) khi \(y=\left[\begin{matrix}=5\\=-5\end{matrix}\right.\).