Theory:

Result \(5\):
If two circles touch internally, the distance between their centres is equal to the difference of their radii.
Explanation:
 
r5.png
 
If  two circles touch internally at a point \(B\), then the distance \(OA\) is equal to the difference of the radii \(OB\) and \(AB\).
 
\(\Rightarrow\) \(OA\) \(=\) \(OB\) \(-\) \(AB\)
Proof for the result:
Let the two circles with centres \(O\) and \(A\) intersect each other internally at the point \(B\).
 
Let the radius \(OB\) \(=\) \(r_{1}\) and \(AB\) \(=\) \(r_{2}\) and \(r_{1}\) \(>\) \(r_{2}\).
 
Let distance between the centres be \(d\).
 
\(\Rightarrow\) \(OA\) \(=\) \(d\)
 
From the figure, we observe that \(OA\) \(=\) \(OB\) \(-\) \(AB\).
 
\(\Rightarrow\) \(d\) \(=\) \(r_{1}\) \(-\) \(r_{2}\)
Example:
Two circle with radii \(4\) \(cm\) and \(5\) \(cm\) intersect at a point \(O\) internally. If so, find the distance between their centres.
 
Solution:
 
By the result, we know that:
 
Distance between the centres \(=\) Difference of the radii.
 
Thus, the distance between the centres \(=\) \(5\) \(cm\) \(-\) \(4\) \(cm\)
 
\(=\) \(1\) \(cm\)