Find the Radius — A Russian 7th Grade Geometry Problem

CHAPTERS 0:00 The problem 0:37 Call the radius x 0:58 Fact one - a radius meets a tangent at 90 degrees 1:30 The rectangle APQD, and the pieces of each side 2:06 Fact two - two tangents from a point are equal 2:53 The slanted side is 25 - 2x 3:27 The clever step - drop a perpendicular from B 4:28 Pythagoras 5:37 The radius is 6