# Relationship between inner and outer radius of a two concentric circles

#### otlconcepts

##### New member
If i have two circles that say 24" apart from each other. one inside the other.
and i know the radius of the inside circel, how can i calculate the outside radius

#### Joppy

##### Well-known member
MHB Math Helper
Did you draw a picture?

\begin{tikzpicture}

\draw[thick,red] (0,0) circle (4);
\draw[->,thick,red] (0,0) -- (4, 0) node[below, xshift=-1cm] {$r_2$};
\draw[thick,blue] (0,0) circle (2);
\draw[->, thick, blue] (0,0) -- (2, 0) node[below, xshift=-0.75cm] {$r_1$};

\end{tikzpicture}

$r_2 - r_1 = 24"$ right? And you know $r_1$.

Last edited:

#### MarkFL

Suppose $$r$$ is the radius of the inner circle and $$R$$ is the radius of the inner circle, where $$k$$ is the difference between the two radii:
$$\displaystyle R-r=k$$