# [Tex/LaTex] Drawing an arc for given two endpoints, center and radius of the arc

arctikz-pgf

Consider the following code:

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{intersections, calc}

\begin{document}
\begin{tikzpicture}[auto]
\draw (0,0) node[below] {$P$} -- (2,0) node[below] {$Q$};
\path[name path=circ] (3,4) node[above]  {$O$} circle (3.5);
\path[name path=l1] (0,0) -- (0, 4);
\path[name path=l2] (2,0) -- (2,4);

\path[name intersections={of=l1 and circ, by=a}];
\path[name intersections={of=l2 and circ, by=b}];
\draw (0,0) -- (a) node[above] {$R$};
\draw (2,0) -- (b) node[above] {$X$};

\draw[densely dotted] (3,4) -- (3,0) node[right] {$Z$} -- (2,0);
\draw[densely dotted] (b) -- (b -| 3,0) node[right] {$Y$} (a) -- (a -| 3,0) node[right] {$S$};
\draw[densely dotted] (a) -- (3,4) -- (b);

\draw[|<->|] (-0.1, 0) -- node {$H$} ($(a) +(-.1,0)$);
\draw[|<->|] (2.1,0) -- node[swap] {$h$} ($(b) +(.1,0)$);

\draw[|<->|] (0, .1) --node {$d$} (2,.1);

\end{tikzpicture}
\end{document}


What I want to do is, to draw an arc where two endpoints are R and X, and the center of the arc is O. How can I do this by not manually calculating the angle of the two lines OR and OX?

You can use the angles-library of tikz.

Add angles to your \usetikzlibrary

\usetikzlibrary{intersections, calc, angles}


Add at the end of your code:

\coordinate (o) at (3,4);
\pic [draw, angle radius=3.5cm] {angle=a--o--b};


The points R and X are already defined by the two intersection points a and b. You just need to add a coordinate at your point O to let tikz do the work.

EDIT: Complete example with explicit coordinates

\documentclass[tikz, border=5mm]{standalone}

\usetikzlibrary{intersections, calc, angles}

\begin{document}
\begin{tikzpicture}
\coordinate [label=below:$P$] (p) at (0,0);
\coordinate [label=below:$Q$] (q) at (2,0);
\coordinate [label=above:$O$] (o) at (3,4);

\path [name path=l1] (p) -- +(0,4);
\path [name path=l2] (q) -- +(0,4);
\path [name path=circ] ($(o)+(200:3.5cm)$) arc (200:260:3.5cm);

\path [name intersections={of=l1 and circ, by=ri}];
\path [name intersections={of=l2 and circ, by=xi}];

\coordinate [label=above:$R$] (r) at (ri);
\coordinate [label=above:$X$] (x) at (xi);

\coordinate [label=right:$S$] (s) at (r-|o);
\coordinate [label=right:$Y$] (y) at (x-|o);
\coordinate [label=below:$Z$] (z) at (q-|o);

\draw (r) -- (p) -- (q) -- (x);

\begin{scope}[dotted]
\draw (r) -- (s) (q) -- (z) -- (o) (x) -- (y) (r) -- (o) -- (x);
\pic [draw, angle radius=3.5cm] {angle=r--o--x};
\end{scope}

\begin{scope}[|<->|]
\draw ([xshift=-.1cm]r) -- ([xshift=-.1cm]p) node [midway, left] {$H$};
\draw ([xshift=.1cm]q) -- ([xshift=.1cm]x) node [midway, right] {$h$};
\draw ([yshift=.1cm]p) -- ([yshift=.1cm]q) node [midway, above] {$d$};
\end{scope}
\end{tikzpicture}
\end{document}