Prima di ogni figura ricordare di mettere
\documentclass[usenames,dvipsnames]{article}
\usepackage{pgfplots}
Prima figura
codice prima figura
\begin{center}\begin{tikzpicture}
\begin{axis}[
axis equal image,
axis x line=center,
axis y line=center,
xmin = -20,
xmax = 20,
ymin = -16,
ymax = 16,
xlabel = {$x$},
ylabel = {$y$},
extra x tick label = {$\hspace{-0.6cm}0$},
y label style={anchor=north east}
]
\addplot[mark=*,very thick,red] coordinates {(-2,-1) (0,8) (6,1)};
\addplot[mark=*] coordinates {(-2,-1)} node[below left] {$A$};
\addplot[mark=*] coordinates {(6,1)} node[above right] {$B$};
\addplot[mark=*] coordinates {(0,8)} node[below left] {$P$};
\end{axis}
\end{tikzpicture}\end{center}
seconda figura
codice seconda figura
\begin{center}\begin{tikzpicture}
\begin{axis}[
axis equal image,
axis x line=center,
axis y line=center,
xmin = -20,
xmax = 20,
ymin = -16,
ymax = 16,
xlabel = {$x$},
ylabel = {$y$},
extra x tick label = {$\hspace{-0.6cm}0$},
y label style={anchor=north east}
]
\addplot[very thick,domain=-10:10,samples=100,red] {-2*x};
\addplot[mark=*,very thick,red] coordinates {(-2,-1) (4,-8) (6,1)};
\addplot[mark=*] coordinates {(-2,-1)} node[below left] {$A$};
\addplot[mark=*] coordinates {(6,1)} node[above right] {$B$};
\addplot[mark=*] coordinates {(4,-8)} node[below left] {$Q$};
\addplot[mark=*] coordinates {(4,-8)} node[above right] {$y=-2x$};
\end{axis}
\end{tikzpicture}\end{center}
terza figura
codice terza figura
\begin{center}\begin{tikzpicture}
\begin{axis}[
axis equal image,
axis x line=center,
axis y line=center,
xmin = -20,
xmax = 20,
ymin = -16,
ymax = 16,
xlabel = {$x$},
ylabel = {$y$},
extra x tick label = {$\hspace{-0.6cm}0$},
y label style={anchor=north east}
]
\addplot[mark=*,very thick,red] coordinates {(-2,-1) (0,8) (6,1) (4,-8) (-2,-1) };
\addplot[mark=*] coordinates {(-2,-1)} node[below left] {$A$};
\addplot[mark=*] coordinates {(4,-8)} node[below left] {$Q$};
\addplot[mark=*] coordinates {(6,1)} node[above right] {$B$};
\addplot[mark=*] coordinates {(0,8)} node[below left] {$P$};
\end{axis}
\end{tikzpicture}\end{center}