# Tikz: specifying radius of circle using two points without Euclid package

A slight modification of the first attempt works. The second one works as well if you add the missing in (and one may want to switch to the modern syntax circle[radius=<radius>]).

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc}

\begin{document}

Attempt 1 slightly modified:

\begin{center}
\begin{tikzpicture}
\coordinate (a) at (0,0);
\coordinate (b) at (1.3,1.5);
\draw let \p1=($(b)-(a)$),\n1={veclen(\x1,\y1)} in (a) circle[radius=\n1] ;
\end{tikzpicture}
\end{center}

\begin{center}
\begin{tikzpicture}[]
\coordinate (a) at (0,0);
\coordinate (b) at (1.3,1.5);
% https://tex.stackexchange.com/a/58287
\draw
let
\p1 = (a),
\p2 = (b),
\n1 = {veclen((\x2-\x1),(\y2-\y1))} in
\end{tikzpicture}
\end{center}

\end{document}


With tikzlibrary through, it can be done easily:

\node [draw] at (a) [circle through={(b)}] {};


\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{through}
\begin{document}
\begin{tikzpicture}
\draw[help lines] (-2,-2) grid (2,2);
\node[circle,fill=red,inner sep=1pt] (a) at (0,0){a};
\node[circle,fill=red,inner sep=1pt] (b) at (1.3,1.5){b};

\node [draw,thick] at (a) [circle through={(b)}] {};
\end{tikzpicture}
\end{document}