The document proves that the alternate segment theorem, which states that any angle in an alternate segment of a circle is equal to an angle formed by the tangent and chord at the point of contact. It does this by joining a radius to the point of contact P and extending it to meet the circle at Z. It then uses properties of angles on tangents, in semicircles, and angle sums in triangles to show that ∠APY = ∠ABP.