Euclid's Elements: Book 3, Prop 36

Mr Burnham's step by step board presentation of Book 3, Proposition 36: "If a point is taken outside a circle and two straight lines fall from it on the circle, and if one of them cuts the circle and the other touches it, then the rectangle contained by the whole of the straight line which cuts the circle and the straight line intercepted on it outside between the point and the convex circumference equals the square on the tangent."