Problem
GEO-B1-M05-P005 Radius to a Tangent
#5
★☆☆☆☆ Level 1 of 5
Line \(l\) is tangent to a circle with centre \(O\) at point \(A\). Prove that \(OA\perp l\).
Use the basic fact: the radius to the point of tangency is perpendicular to the tangent.
By the tangent theorem, the radius drawn to the point of tangency is perpendicular to the tangent. Since \(A\) is the point of tangency, \(OA\perp l\).
This basic fact is needed for all later tangent problems.