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?

Because $D$ lies on the circle, $OD^2=OB^2+BD^2$. Also because $OD(=OC)=OB+BC$, $OB=OD-BC$. Substituting $OB$ in the first equation yields
$ $ $ $ $ $ $ $ $OD^2=(OD-BC)^2+BD^2$
$≡$ $OD^2= OD^2-2\centerdot OD\centerdot BC+ BC^2+BD^2$
$≡$ $2\centerdot OD\centerdot BC =BC^2+BD^2$
$≡$ $OD = (BC^2 + BD^2)/(2\centerdot BC)$  
For the example at hand, this results in a radius of $(2^2+4^2)/(2\centerdot 2) = 5$.
