Stage 18 · Circles

18.5.1 A Tangent's Property: The Radius Must Be Perpendicular

18.5 Tangent Lines — The Line That Only Kisses Once

Point 1 of 5

18.5.1 A Tangent's Property: The Radius Must Be Perpendicular

Core idea

When a line touches a circle at just one point, the radius drawn to that point is always perpendicular to the line.

Module goal. Dig deep into the borderline case of tangency, mastering a tangent's properties, its test, and the tangent-length theorem, then apply them to a triangle's inscribed circle.

eastmath.com · 18.5 Tangent Lines — The Line That Only Kisses Once · 18.5.1 A Tangent's Property: The Radius Must Be Perpendicular