# Draw an ellipse into two parts using tikzpicture environment

I know this is a VERY BASIC question but I can't finish the other part of this ellipse using dashed points:

MWE:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{fancyhdr}
\usepackage{amssymb}
\usepackage{amsthm}

\usepackage{pgfplots}
\usetikzlibrary{arrows.meta, patterns}
\pgfplotsset{compat=1.8}
\pgfplotsset{soldot/.style={color=black,only marks,mark=*}}
\pgfplotsset{holdot/.style={color=red,fill=white,very thick,only marks,mark=*}}
\usepackage{amsmath}

\usepackage{mathtools}

\setpapersize{A4}
\setmargins{2.2cm}
{0.5cm}
{16.5cm}
{23.42cm}
{30pt}
{1cm}
{0pt}
{2cm}
\pagestyle{fancy}
\fancyhf{}
\cfoot{\large \thepage}

\begin{document}

\begin{center}
\begin{tikzpicture}[scale=1.5]
\begin{axis}[
legend pos=outer north east,
axis lines = center,
label style={font=\tiny},
legend style={font=\tiny},
xticklabel style = {font=\tiny},
yticklabel style = {font=\tiny},
xmin=-2.5,
ymin=-1.5,
xmax=2.5,
ymax=1.5,
xlabel = $x$,
ylabel = $y$,
clip=false,
legend style={cells={align=left}}
]
\addplot[domain=-pi/3:pi/3, samples=80,red,thick] ({2*cos((x) r)}, {sin((x) r)});
\addplot[soldot] coordinates {(1,0.866)} node[above] {\tiny $\left(1,\frac{\sqrt 3}2\right)$};
\addplot[soldot] coordinates {(1,-0.866)} node[below] {\tiny $\left(1,-\frac{\sqrt 3}2\right)$};
\end{axis}
\end{tikzpicture}
\end{center}

\end{document}


The equation is x^2/4 + y^2 = 1.

Thank you!

• What is the question? – Dr. Manuel Kuehner Jul 18 '18 at 22:24
• @Dr.ManuelKuehner how to complete the left side of the ellipse (using red dashed points). – manooooh Jul 18 '18 at 22:31
• Do you have the equation for the line? – Dr. Manuel Kuehner Jul 18 '18 at 22:31

Why not continuing what you have already?

\documentclass[]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.16}
\begin{document}
\begin{tikzpicture}[scale=1.5]
\begin{axis}[
legend pos=outer north east,
axis lines = center,
label style={font=\tiny},
legend style={font=\tiny},
xticklabel style = {font=\tiny},
yticklabel style = {font=\tiny},
xmin=-2.5,
ymin=-1.5,
xmax=2.5,
ymax=1.5,
xlabel = $x$,
ylabel = $y$,
clip=false,
legend style={cells={align=left}}
]
\addplot[domain=-pi/3:pi/3, samples=10,red,thick] ({2*cos((x) r)}, {sin((x) r)})
node[pos=1,circle,fill=black,inner sep=1pt,label={[text=black,font=\tiny]90:{$\left(1,\frac{\sqrt 3}2\right)$}}] {}
node[pos=0,circle,fill=black,inner sep=1pt,label={[text=black,font=\tiny]-90:{$\left(1,-\frac{\sqrt 3}2\right)$}}] {};
\addplot[dashed,domain=pi/3:5*pi/3, samples=10,red,thick] ({2*cos((x) r)}, {sin((x) r)});
\end{axis}
\end{tikzpicture}
\end{document}


I've decreased the sample number. Try to place embellishments using TikZ machinery instead of extra plots.

• Well, perfect. :) You also got rid of the strange "annotation" approach. – Dr. Manuel Kuehner Jul 18 '18 at 22:32
• Perfect! This is what I want. I did not realized that how to continue. – manooooh Jul 18 '18 at 22:35
• @Dr.ManuelKuehner what do you mean with "strange "annotation" approach"? – manooooh Jul 18 '18 at 22:36
• @manooooh: \addplot[soldot] coordinates {(1,0.866)} node[above] {\tiny $\left(1,\frac{\sqrt 3}2\right)$}; --> empty addplot but that's just a side note. – Dr. Manuel Kuehner Jul 18 '18 at 22:38
• Very nice! +1. Could you perhaps consider adding smooth to the plots? – user121799 Jul 19 '18 at 5:05