\(\begin{array}{l}
a)\quad y = 2x + \sqrt{25 - x^2}\\
TXĐ: D = [-5;5]\\
\quad y' = 2 - \dfrac{x}{\sqrt{25 - x^2}}\\
y' = 0 \Leftrightarrow \dfrac{x}{\sqrt{25 - x^2}} = 2 \Leftrightarrow x = 2\sqrt5\\
\text{Bảng biến thiên:}\\
\begin{array}{|c|cr|}
\hline
x & -5 & & & 2\sqrt5 & & & 5\\
\hline
y' &||& +& & 0\quad & - & &||\\
\hline
&&&&5\sqrt5\\
y & &\nearrow& & &\searrow\\
&-10&&&&&&10\\
\hline
\end{array}\\
b)\quad y = 2x - \sqrt{16 - x^2}\\
TXĐ: D = [-4;4]\\
\quad y' = 2 + \dfrac{x}{\sqrt{16 - x^2}}\\
y' = 0 \Leftrightarrow \dfrac{x}{\sqrt{16 - x^2}} =- 2 \Leftrightarrow x = -\dfrac{8}{\sqrt5}\\
\text{Bảng biến thiên:}\\
\begin{array}{|c|cr|}
\hline
x & -4 & & & -\dfrac{8}{\sqrt5} & & & 4\\
\hline
y' &||& -& & 0\quad & + & &||\\
\hline
&-8&&&&&&8\\
y & &\searrow& & &\nearrow\\
&&&&-4\sqrt5\ \\
\hline
\end{array}\\
\end{array}\)
