The problem is to find out the solid angle ω subtended by a disc of radius a at a point P distant z from its centre along its axis. α is the semi-vertical angle of the disc at the point P in question. The answer is supposed to be ω = 2π (1 - cos α), according to an online text. However, I find that ω = (Area) / (perpendicular distance)2 = (πa2)/z2 = π tan2α. I mark my answer in red, in contrast to the "correct" answer in blue. Any help?