First, we can draw a picture to understand more easily what the exercise asks of us.
We must find the radius of the circle and this is precisely the distance from the point (5,2) to the origin. Then using the Distance Formula, we have:
![\begin{gathered} d=\sqrt[]{(x_2-x_1)^2+(y_2-y_1)^2} \\ d=\sqrt[]{(5-0)^2+(2-0)^2} \\ d=\sqrt[]{(5)^2+(2)^2} \\ d=\sqrt[]{25+4} \\ d=\sqrt[]{29} \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/high-school/sqx176i47aj3pen9zyv00chyh1vv2jn828.png)
Where

However, if we take it the other way around, we arrive at the same answer. Finally, from what is shown in the previous graph
![\begin{gathered} r=d \\ r=\sqrt[]{29} \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/high-school/4ib9xi12jho0wzdzmum96fygi8lzl2blwm.png)