Euclid's Elements: Book 3, Prop 16

Mr Burnham's step by step board presentation of Book 3, Proposition 16: "The straight line drawn at right angles to the diameter of a circle from its end will fall outside the circle, and into the space between the straight line and the circumference another straight line cannot be interposed, further the angle of the semicircle is greater, and the remaining angle less, than any acute rectilinear angle."