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 ...

A tangent meets a circle at one point and one point only. The angle between a tangent and a radius is a right angle of \(90^\circ\). PT is a tangent and CT is a radius, therefore \(\angle CTP=90^\circ ...