Question 50
In this diagram a scheme is indicated for associating all the points of segment $AB$ with those of segment $A'B',$ and reciprocally. To describe this association scheme analytically, let $x$ be the distance from a point $P$ on $AB$ to $D$ and let $y$ be the distance from the associated point $P'$ of $A'B'$ to $D'.$ Then for any pair of associated points, if $x=a,$ $x+y$ equals:
