Given :
Center of circle, C = ( 1 , 5 )
Point on circle, P = ( 5 , 1 )
To Find :
The radius of a circle.
Solution :
We know, radius of circle is the distance between center and point of circle.
Distance between two points A(p,q) and B(r,s) is given by :

So, distance between the points (1, 5) and (5, 1) is :

Therefore, radius of circle is 5.66 units.
Hence, this is the required solution.