\( \definecolor{colordef}{RGB}{249,49,84} \definecolor{colorprop}{RGB}{18,102,241} \)

Homothety

What is a Homothety?

Definition Homothety of a Point
The homothety of point \(M\) with center \(O\) and scale factor \(\textcolor{olive}{k}\) is the point \(M'\) such that \(\overrightarrow{OM'}=\textcolor{olive}{k}\overrightarrow{OM}\). In particular, the points \(O\), \(M\), and \(M'\) lie on the same line, and the distance from \(O\) to \(M'\) is \(|\textcolor{olive}{k}|\) times the distance from \(O\) to \(M\).

\(\dfrac{\textcolor{colorprop}{OM'}}{\textcolor{colordef}{OM}}=|\textcolor{olive}{k}|\)
Example
Find the coordinates of the image of point \(M\) under a homothety with center \(O\) and scale factor \(k=2\).

  • \(\overrightarrow{OM}\):
  • \(\overrightarrow{OM'}=\textcolor{olive}{2}\overrightarrow{OM}\):
  • \(M'(\textcolor{colordef}{5},\textcolor{colorprop}{3})\)

Definition Homothety
The homothety of a figure with center \(O\) and scale factor \(k\) is the image obtained by applying the homothety with center \(O\) and scale factor \(k\) to all its points.
Example
The figure \(\textcolor{colorprop}{A'}\) is the image of figure \(\textcolor{colordef}{A}\) under a homothety with center \(O\) and scale factor \(2\).