The radius of a circle is always perpendicular to the tangent at the point of contact

Radius of a circle is perpendicular to the tangent at the point of contact

The above theorem of a circle is extremely useful in finding the equation of tangent of a circle and also for a number of other things.

Stated simply, it means that whenever a tangent is drawn to a circle, it touches the circle at a point called the point of contact. If you draw the radius of the circle from its center to the point at which the tangent touches the circle (that is, from the center to the point of contact), the radius will be perpendicular to the tangent. That is, the radius and tangent will form a 90 degree angle.