Finding a Radius of a Tangent Sphere (2013 AMC 12 A Problem 18)

From a channel Lumen includes in full: LetsSolveMathProblems works through competition maths problems (Putnam, olympiads) and university topics in full, with rigorous explanations.

A 6-minute video from LetsSolveMathProblems's catalogue; the title names its topic.

Video by LetsSolveMathProblems, played through YouTube's embedded player.