A perpendicular bisector of a line segment is a line which is perpendicular to this line and passes through its midpoint. This drawing shows perpendicular bisectors of a triangle. They meet in the center of the circumcircle of the triangle.
This example was written by Sam Britt answering a question on TeX.SE.

Edit and compile if you like:
% Perpendicular bisectors of a triangle% Author: Sam Britt\documentclass[tikz,border=10pt]{standalone}\usetikzlibrary{calc}\begin{document}\begin{tikzpicture}[scale=3,>=stealth,point/.style = {draw, circle, fill = black, inner sep = 1pt},dot/.style = {draw, circle, fill = black, inner sep = .2pt},]% the circle\def\rad{1}\node (origin) at (0,0) [point, label = {below right:$P_c$}]{};\draw (origin) circle (\rad);% triangle nodes: just points on the circle\node (n1) at +(60:\rad) [point, label = above:$1$] {};\node (n2) at +(-145:\rad) [point, label = below:$2$] {};\node (n3) at +(-45:\rad) [point, label = {below right:$3$ $(0, 0, 0)$}] {};% triangle edges: connect the vertices, and leave a node at the midpoint\draw[->] (n3) -- node (a) [label = {above right:$\vec{v}_1$}] {} (n1);\draw[->] (n3) -- node (b) [label = {below right:$\vec{v}_2$}] {} (n2);\draw[dashed] (n2) -- (n1);% Bisectors% start at the point lying on the line from (origin) to (a), at% twice that distance, and then draw a path going to the point on% the line lying on the line from (a) to the (origin), at 3 times% that distance.\draw[dotted]($ (origin) ! 2 ! (a) $)node [right] {Bisector 1}-- ($(a) ! 3 ! (origin)$ );% similarly for origin and b\draw[dotted]($ (origin) ! 2 ! (b) $)
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