# Drawing a circle through 3 non-collinear points

Does circle through works with 3 points:

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{calc,through}
\begin{document}
\begin{tikzpicture}
\coordinate (A) at (1,1);
\coordinate (B) at (2,2);
\coordinate (C) at (3,1.5);

\node[draw,line width=2pt] [circle through={(A)(B)(C)}] {};

\foreach \i in {A,B,C} {
\node[circle,minimum size=1pt,fill=red] at(\i) {};
}
\end{tikzpicture}
\end{document}


I want to draw a circle through A,B and C.

The tkz-euclide package has a macro to do this. The manual is written in French.

1. First, we define the circle with the macro \tkzDefCircle.
2. This macro returns two values that are the center recovered with the macro \tkzGetPoint{O}
3. and the radius that is recovered with the macro \tkzGetLength{rayon}.

Once this is done, we draw the circle with the macro \tkzDrawCircle[R](O,\rayon pt)

\documentclass[tikz,border=5mm]{standalone}
%\usepackage{tikz}
\usepackage{tkz-euclide}
\usetikzlibrary{calc,through}
\begin{document}
\begin{tikzpicture}
\coordinate (A) at (1,1);
\coordinate (B) at (2,2);
\coordinate (C) at (3,1.5);

%    \node[draw,line width=2pt] [circle through={(A)(B)(C)}] {};

\tkzDefCircle[circum](A,B,C)
\tkzGetPoint{O} \tkzGetLength{rayon}
\tkzDrawCircle[R](O,\rayon pt)

\foreach \i in {A,B,C} {
\node[circle,minimum size=1pt,fill=red] at(\i) {};
}
\end{tikzpicture}
\end{document}


A new node style, based on the derivation below plus the information that one can use intersection of \p1--\p3 and \p2--\p4, which I learned from AndréC's nice answer

\documentclass[tikz,border=5mm]{standalone}
\usetikzlibrary{calc,through}
\tikzset{circle through 3 points/.style n args={3}{%
insert path={let    \p1=($(#1)!0.5!(#2)$),
\p2=($(#1)!0.5!(#3)$),
\p3=($(#1)!0.5!(#2)!1!-90:(#2)$),
\p4=($(#1)!0.5!(#3)!1!90:(#3)$),
\p5=(intersection of \p1--\p3 and \p2--\p4)
in },
at={(\p5)},
circle through= {(#1)}
}}

\begin{document}
\begin{tikzpicture}
\coordinate (A) at (1,1);
\coordinate (B) at (2,2);
\coordinate (C) at (3,1.5);
\node[circle through 3 points={A}{B}{C},draw=blue]{};
\foreach \i in {A,B,C} {
\node[circle,minimum size=1pt,fill=red] at(\i) {};
}
\end{tikzpicture}
\end{document}


Just for fun: an analytic solution based on calc only. (My personal opinion, though, is that this method is more "TikZy", i.e. closer to how the standard TikZ styles work, than the tkz-euclide macros, which are more like pstricks, which I have left behind. However, this is just a personal opinion, and might not be shared by others.)

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{calc}
\tikzset{circle through 3 points/.style n args={3}{%
insert path={let \p1=($(#1)-(#2)$),\p2=($(#1)!0.5!(#2)$),
\p3=($(#1)-(#3)$),\p4=($(#1)!0.5!(#3)$),\p5=(#1),\n1={(-(\x2*\x3) + \x3*\x4 + \y3*(-\y2 +
\y4))/(\x3*\y1 - \x1*\y3)},\n2={veclen(\x5-\x2-\n1*\y1,\y5-\y2+\n1*\x1)} in
({\x2+\n1*\y1},{\y2-\n1*\x1}) circle (\n2)}
}}
\begin{document}
\begin{tikzpicture}
\coordinate (A) at (1,1);
\coordinate (B) at (2,2);
\coordinate (C) at (3,1.5);
\draw[circle through 3 points={A}{B}{C}];
\foreach \i in {A,B,C} {
\node[circle,minimum size=1pt,fill=red] at(\i) {};
}
\end{tikzpicture}
\end{document}


(Note that \n1 is a fraction, and could in principle not be well defined. If you ever encounter this case, just change the ordering, e.g. do \draw[circle through 3 points={B}{C}{A}]; or something along those lines.)

ADDENDUM: Explanation of the analytic formula.

\documentclass[tikz,border=3.14mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{calc}
\tikzset{circle through 3 points/.style n args={3}{%
insert path={let \p1=($(#1)-(#2)$),\p2=($(#1)!0.5!(#2)$),
\p3=($(#1)-(#3)$),\p4=($(#1)!0.5!(#3)$),\p5=(#1),\n1={(-(\x2*\x3) + \x3*\x4 + \y3*(-\y2 +
\y4))/(\x3*\y1 - \x1*\y3)},\n2={veclen(\x5-\x2-\n1*\y1,\y5-\y2+\n1*\x1)} in
({\x2+\n1*\y1},{\y2-\n1*\x1}) circle (\n2)}
}}
\begin{document}
\foreach \X in {1,...,5}
{\begin{tikzpicture}[font=\sffamily]
\path[use as bounding box] (-1,-4) rectangle (6,4);
\coordinate (A) at (1,1);
\coordinate (B) at (2,2);
\coordinate (C) at (3,1.5);
\foreach \i in {A,B,C} {
\node[circle,minimum size=1pt,fill=red] at(\i) {};
}
\ifnum\X=1
\node[anchor=north,text width=7cm] (start) at (2.5,0){Starting point: 3 points.};
\foreach \Y in {A,B,C}
{\draw[-latex] (start) to[out=90,in=-90] (\Y) node[above=2pt]{\Y}; }
\fi
\ifnum\X=2
\coordinate (auxAB) at ($(A)!.5!(B)$);
\coordinate (auxBC) at ($(B)!.5!(C)$);
\draw (A) -- (B) -- (C);
\draw ($(auxAB)!1.2cm!90:(B)$) -- ($(auxAB)!1.2cm!-90:(B)$) coordinate(aux1);
\draw ($(auxBC)!1.2cm!90:(B)$) coordinate(aux2) -- ($(auxBC)!1.2cm!-90:(B)$);
\node[anchor=north,text width=7cm] (int) at (2.5,0){The center of the circle is
where the lines that run through and are orthogonal to the edges intersect.};
\draw[-latex] (int) to[out=45,in=-90] (aux1);
\draw[-latex] (int) to[out=135,in=-90] (aux2);
\fi
\ifnum\X=3
\coordinate[label=below:$P_2$] (auxAB) at ($(A)!.5!(B)$);
\coordinate[label=below:$P_4$] (auxBC) at ($(B)!.5!(C)$);
\foreach \Y in {auxAB,auxBC}
{\fill (\Y) circle (1pt);}
\draw (A) -- (B) -- (C);
\draw ($(auxAB)!1.2cm!90:(B)$) -- ($(auxAB)!1.2cm!-90:(B)$);
\draw ($(auxBC)!1.2cm!90:(B)$) -- ($(auxBC)!1.2cm!-90:(B)$);
\node[anchor=north,text width=7cm] (int) at (2.5,0){Call the points in the
middle $P_2$ and $P_4$, and the differences $P_1=A-B$ and $P_3=B-C$. Then the
orthogonal lines will fulfill
$\gamma_1(\alpha)~=~\left(\begin{array}{c} x_2+\alpha\,y_1\\ y_2-\alpha\,x_1\\ \end{array}\right)$
and
$\gamma_2(\beta)~=~\left(\begin{array}{c} x_4+\beta\,y_3\\ y_4-\beta\,x_3\\ \end{array}\right)\;.$
};
\fi
\ifnum\X=4
\coordinate[label=below:$P_2$] (auxAB) at ($(A)!.5!(B)$);
\coordinate[label=below:$P_4$] (auxBC) at ($(B)!.5!(C)$);
\foreach \Y in {auxAB,auxBC}
{\fill (\Y) circle (1pt);}
\draw (A) -- (B) -- (C);
\draw ($(auxAB)!1.2cm!90:(B)$) -- ($(auxAB)!1.2cm!-90:(B)$);
\draw ($(auxBC)!1.2cm!90:(B)$) -- ($(auxBC)!1.2cm!-90:(B)$);
\node[anchor=north,text width=7cm] (int) at (2.5,0){The center of the circle is
then simply determined by
$\gamma_1(\alpha)~=~\gamma_2(\beta)\;,$
which has the solution
$\alpha~=~\frac{-(x_2\cdot x_3) + x_3\cdot x_4 + y_3\cdot (y_4-y_2 )}{x_3\cdot y_1 - x_1\cdot y_3}\;.$
This is \texttt{\textbackslash n1} in the Ti\emph{k}Z style \texttt{circle through 3 points}.
};
\fi
\ifnum\X=5
\draw[circle through 3 points={A}{B}{C}];
\node[anchor=north,text width=7cm] (int) at (2.5,-0.1){Once we have the center,
determining the radius (\texttt{\textbackslash n2}) is trivial, and we can draw
the circle with a simple \texttt{insert path}.};
\fi
\end{tikzpicture}}
\end{document}


• (+1) Give the analytical expression also, if possible :) – nidhin Nov 21 '18 at 19:16
• @nidhin It is in the code, isn't it? – user121799 Nov 21 '18 at 19:20
• Yes it is there. I meant outside the code. Mathematical expression. – nidhin Nov 21 '18 at 19:25
• @nidhin Done. There is a simple animation that you can create with convert -delay 800 -loop 0 -density 300 -alpha remove <pdf> <gif>, where pdf is the pdf file that gets created if you compile the lower code, and gif the name of the animated gif you will create. – user121799 Nov 21 '18 at 19:54
• @blackened This method creates a node of circle shape, and the center can be extracted with <node>.center, e.g. \node[circle through 3 points={A}{B}{C},draw=blue] (n){}; \draw[latex-] (n.center) -- ++ (-30:1); – user121799 Dec 31 '18 at 6:48

Just for fun with @AndréC's answer:

\documentclass[tikz,border=5mm]{standalone}
\usetikzlibrary{calc,through}
\begin{document}
\begin{tikzpicture}
\coordinate (A) at (1,1);
\coordinate (B) at (2,2);
\coordinate (C) at (3,1.5);

\draw let \p1=($(A)!0.5!(B)$),
\p2=($(A)!0.5!(C)$),
\p3=($(\p1)!2!-90:(B)$),
\p4=($(\p1)!2!90:(B)$),
\p5=($(\p2)!2!-90:(C)$),
\p6=($(\p2)!2!90:(C)$),
\p7=(intersection of \p3--\p4 and \p5--\p6)
in
(A) -- (B)
(A) -- (C)
(\p3) -- (\p4)
(\p5) -- (\p6)
foreach \j in {1,...,7} {
node[circle,minimum size=2pt,fill=red,inner sep=0,label=\j] at(\p\j) {}
}
node[draw,line  width=1pt,circle through= {(A)}] at (\p7)  {};

\foreach \i in {A,B,C} {
\node[circle,minimum size=5pt,fill=red,inner sep=0,label=\i] at(\i) {};
}
\end{tikzpicture}
\end{document}


You can use tkz-euclide like this:

\documentclass{standalone}
\usepackage{tikz}
\usepackage{tkz-euclide}
\usetikzlibrary{calc,through}
\begin{document}
\begin{tikzpicture}
\coordinate (A) at (1,1);
\coordinate (B) at (2,2);
\coordinate (C) at (3,1.5);

\node[draw,line width=2pt] [circle through={(A)(B)(C)}] {};

\foreach \i in {A,B,C} {
\node[circle,minimum size=1pt,fill=red] at(\i) {};
}
\tkzCircumCenter(A,B,C)\tkzGetPoint{O}
\tkzDrawCircle(O,A)
\end{tikzpicture}
\end{document}


(Modified from https://tex.stackexchange.com/a/16024/8650)

If you chose to use tkz-euclide, then you should consider to do all of your drawing with it - depending on what it is - you can e.g. define your points with \tkzDefPoint(x,y).

Just for comparison purpose.

\documentclass[pstricks]{standalone}
\usepackage{pst-eucl}
\begin{document}
\foreach \i in {1.0,1.2,...,4.0}{
\begin{pspicture}(-5,-5)(5,5)
\pstTriangle(4;30){A}(4;90){B}(\i;-45){C}
\pstCircleABC{A}{B}{C}{O}
\end{pspicture}}
\end{document}


The code

\node [draw] at (1,1) [circle through={(A)}] {};


draw a circle whose center is at (1,1) and passes through A. In this case the center of the Circumscribed circle has to be calculated before using through.

I just used the Straightedge and compass construction to calculate the center and then drew circle. The basic idea is that all the perpendicular bisectors of the edges of a triangle meet at the same point: the circumcenter.

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{through,intersections}
\begin{document}
\begin{tikzpicture}
\coordinate (A) at (1,1);
\coordinate (B) at (2,2);
\coordinate (C) at (3,1.5);
\path[name intersections={of = c1 and c2}];
\path[name path=o1] (intersection-1)--(intersection-2);
\path[name intersections={of = c2 and c3}];
\path[name path=o2] (intersection-1)--(intersection-2);
\path[name intersections={of = o1 and o2}];
\node[draw,line width=2pt] at (intersection-1) [circle through={(A)}]{};

\foreach \i in {A,B,C} {
\node[circle,minimum size=1pt,fill=red] at(\i) {};
}
\end{tikzpicture}
\end{document}


Just for fun, another solution inspired by the @marmot solution that calculates the intersection of two defined perpendicular bisector with the calc library.

\documentclass[tikz,border=5mm]{standalone}
\usetikzlibrary{calc,through}
\tikzset{circle through 3 points/.style n args={3}{%
insert path={let    \p1=($(#1)!0.5!(#2)$),
\p2=($(#1)!0.5!(#3)$),
\p3=($(#1)!0.5!(#2)!1!-90:(#2)$),
\p4=($(#1)!0.5!(#3)!1!90:(#3)$),
\p5=(intersection of \p1--\p3 and \p2--\p4)
in
node at (\p5) [draw,line  width=2pt,circle through= {(#1)}]{}}
}}

\begin{document}
\begin{tikzpicture}
\coordinate (A) at (1,1);
\coordinate (B) at (2,2);
\coordinate (C) at (3,1.5);
\draw[circle through 3 points={A}{B}{C}];
\foreach \i in {A,B,C} {
\node[circle,minimum size=1pt,fill=red] at(\i) {};
}
\end{tikzpicture}
\end{document}

• Nice! I didn't know that one could use \p1 etc. in intersection of \p1--\p3 and \p2--\p4. (I guess it will be more elegant if you manage to eliminate draw,line width=2pt, from the node since \path[circle through 3 points={A}{B}{C}]; also draws the path, which is not what \path usually does. – user121799 Nov 21 '18 at 21:11
• @marmot It is simple to delete line width=2pt which is a given parameter of the lucky1928 question. On the other hand, it is more difficult to delete draw. To do that, you have to find a way to pass the parameters to the node, I'll get down to work and explore (finally!) the key handlers. I just saw that you succeeded! Congratulations! – AndréC Nov 21 '18 at 21:56
• I really could only do that after I learned the intersection of \p1--\p3 and \p2--\p4 thing from you, which is much better than the analytic computation of the intersection point, and presumably also more stable (even though the analytic derivation was fun ;-). Thanks a lot! (My aim was to create a style that behaves the same way as circle through, which your observation made possible, so thanks again!) – user121799 Nov 21 '18 at 22:03
• @marmot Thank you very much and I have to return the compliment because it is by adapting your solution that I learned how to use insert path and that I am finally motivated to learn key handlers. – AndréC Nov 21 '18 at 22:07
• I really like that you acknowledge other posts (and I try to do the same). I wish all users would do that.... ;-) – user121799 Nov 21 '18 at 22:11