19. Find critical points of \( f(x, y, z)=x y+y z \) with constraints \( x^{2}+y^{2}=8 \) dan \( y z=8 \)

\( \left. \begin{array} { l } { \lim _ { x \rightarrow 3 } ( \frac { x - 3 } { x ^ { 2 } - 9 } ) } \\ \hline x ( x ) \\ \hline - 2.99 \\ \hline \end{array} \right. \)

\( \begin{array} { l } { \lim _ { x \rightarrow 3 } ( \frac { x - 3 } { x ^ { 2 } - 9 } ) } \\ { x } \\ { x ( x ) } \\ { f ( x ) } \\ { - 2.99 } \end{array} - 2.999 | 3 | 3.001 | 3.01 | ) \)

\( \left. \begin{array} { l } { \lim _ { x \rightarrow 0 } ( \frac { \sin x } { x } ) } \\ { \left. \begin{array} { l | l | l | l } { x } & { - 0.001 } & { 0 } & { 0.0011 } \\ \hline f ( x ) & { } & { } & { } \\ \hline \end{array} \right. } \end{array} \right. \)

\( \left. \begin{array} { l } { \lim _ { x \rightarrow 0 } ( \frac { \sin x } { x } ) } \\ { \left. \begin{array} { l | l | l | l | } { x } & { - 0.001 } & { 0 } & { 0.001 } \\ \hline f ( x ) & { } & { } & { } \\ \hline \end{array} \right. } \end{array} \right. \)

(1)- \( \int_{-\infty}^{\infty} \frac{e^{x}}{1+e^{2 x}} \mathrm{dx} ? \)