How can I find the radius of a circle from a chord and a section of the radius? Draw a circle with center O.
Line AD is a chord that is 8cm long. The arc above is smaller than the one below.
B is the center of AD.
Line CB is a line that is 2cm long. It meets AD at 90°.
Diagram:

Given the facts above, is it possible to find CO (the radius of the circle)? If so, what is the radius, and how can I find it for any other circle?

Let $\overline{CC^\prime}$ be a diameter.
By Power of a Point, $AB \cdot BD = CB \cdot BC^\prime = 4 \cdot 4 = 2 \cdot BC^\prime$.
Then $BC^\prime = 8$, $CC^\prime = 10$, and $CO = 5$.
