In Analytical Geometry there is a most useful equation which is very rarely used and seems to be very unfamiliar to candidates in school examinations. It is the equation of the chord of a conic whose mid-point is (x1, y1). Speaking from a long experience as an examiner, I am always astonished at the unfamiliarity of candidates with this particular equation. If one sets a question asking the candidates to obtain this equation and follows this with a simple rider designed to show its usefulness, even if the candidates know how to find the equation, they rarely solve the rider by means of it.