Interactive resource
Tangent properties
Geometry. A tangent is perpendicular to the radius at the point of contact. Free interactive GeoGebra resource — adjust parameters in the browser.
Overview
A tangent is perpendicular to the radius at the point of contact.
Explanation
A tangent meets the radius at the contact point at 90°. If P is the contact and O the center, OP ⊥ tangent.
Criterion: a line is tangent iff its distance from the center equals the radius.
Later tangent-length results build on this right angle.
How to use
Drag contact point P; the angle between OP and the tangent stays 90°.