The radius of a circle is a line, which starts at the centre of the circle and ends on a point on the circumference. The radius is half the length of the diameter. Looking at the diagram, since OA and ...

In the diagram below, AB is the chord of a circle with centre O. OM is perpendicular to AB (meet at a right angle). Look at triangles OAM and OBM. The hypotenuses (OA and OB) are the same, as they are ...