A chord AB of length 300 mm is at a distance of 100 mm from the center of a circle. Find the radius of the circle.

D is the midpoint of the chord AB.
Given $AB = 300$ mm.

Let O be the center of the circle and $r$ be its radius.
Since D is the midpoint of chord AB and $DC \perp AB$, the line segment DC passes through the center O.
C is a point on the circle, so OC is a radius, $OC = r$.
The distance from the center O to the chord AB is OD.
From the diagram, C