Triangle with sides 9, 40 and 41 is a right triangle.
$9^{2}$+$40^{2}$=$41^{2}$
In a right angled triangle, the radius of the circle that circumscribes the triangle is half the hypotenuse.
Therefore, the radius of the circle that circumscribes the triangle = $\frac{41}{2}$ = 20.5