\begin{document}
  \begin{tikzpicture}[domain=0:4]
    \draw[very thin,color=gray] (-0.1,-1.1) grid (3.9,3.9);
    \draw[->] (-0.2,0) -- (4.2,0) node[right] {$x$};
    \draw[->] (0,-1.2) -- (0,4.2) node[above] {$f(x)$};
    \draw[color=red]    plot (\x,\x)             node[right] {$f(x) =x$};
    \draw[color=blue]   plot (\x,{sin(\x r)})    node[right] {$f(x) = \sin x$};
    \draw[color=orange] plot (\x,{0.05*exp(\x)}) node[right] {$f(x) = \frac{1}{20} \mathrm e^x$};
  \end{tikzpicture}
\end{document}

\usepackage{circuitikz}
\begin{document}
	\begin{circuitikz}[american, voltage shift=0.5]
		\draw (0,0)
		to[isource, l=$I_0$, v=$V_0$] (0,3)
		to[short, -*, i=$I_0$] (2,3)
		to[R=$R_1$, i>_=$i_1$] (2,0) -- (0,0);
		\draw (2,3) -- (4,3)
		to[R=$R_2$, i>_=$i_2$]
		(4,0) to[short, -*] (2,0);
	\end{circuitikz}
\end{document}

\usepackage{pgfplots}
\pgfplotsset{compat=1.16}
\begin{document}
	\begin{tikzpicture}
		\begin{axis}[colormap/viridis]
		\addplot3[
			surf,
			samples=18,
			domain=-3:3
		]
		{exp(-x^2-y^2)*x};
		\end{axis}
	\end{tikzpicture}
\end{document}

\usepackage{tikz-cd}

\begin{document}
\begin{tikzcd}

    T
    \arrow[drr, bend left, "x"]
    \arrow[ddr, bend right, "y"]
    \arrow[dr, dotted, "{(x,y)}" description] & & \\
    K & X \times_Z Y \arrow[r, "p"] \arrow[d, "q"]
    & X \arrow[d, "f"] \\
    & Y \arrow[r, "g"]
    & Z

\end{tikzcd}

\quad \quad

\begin{tikzcd}[row sep=2.5em]

A' \arrow[rr,"f'"] \arrow[dr,swap,"a"] \arrow[dd,swap,"g'"] &&
  B' \arrow[dd,swap,"h'" near start] \arrow[dr,"b"] \\
& A \arrow[rr,crossing over,"f" near start] &&
  B \arrow[dd,"h"] \\
C' \arrow[rr,"k'" near end] \arrow[dr,swap,"c"] && D' \arrow[dr,swap,"d"] \\
& C \arrow[rr,"k"] \arrow[uu,<-,crossing over,"g" near end]&& D

\end{tikzcd}

\end{document}