In a triangle, orthocentre, centroid and circumcenter are collinear and centroid divides orthocenter and circumcenter in 2 : 1 ratio. Here AB = 2 BC $AB = \sqrt {{{\left( {3 + 3} \right)}^2} + {{\left( {3 - 5} \right)}^2}}$ $= \sqrt {36 + 4}$ $= \sqrt {40}$ $= 2\sqrt {10}$ $\therefore\,\,\,\,$ BC $= \sqrt {10}$ $\therefore\,\,\,\,$ AB + BC $= 2\sqrt {10} + \sqrt {10}$ $= 3\sqrt {10}$ AC is the diameter of the circle. $\therefore\,\,\,$ Radius $= {1 \over 2}\,\,$ AC $= {1 \over 2} \times 3\sqrt {10}$ $= 3\sqrt {{5 \over 2}}$