Given are two parallel lines a, b and two points on each: (A,A1) and (B,B1) respectively. For any real number x, construct the points Ax = (1-x)*A + x*A1, Bx = (1-x)*B + x*B1. The segment AxBx depends on x, and has the following properties. [1] AAx/AxA1 = BBx/BxB1 for every x. [2] Lines Lx = AxBx pass through the intersection point C of AB and A1B1. [3] AxBx/BxC is constant i.e. independent of x.
All properties are trivial consequences of Thale's theorem. The figure here is a preamble to the much more interesting figure, resulting by retaining all definitions except the parallelity of lines a and b. This is studied in LinesOfSegments2.html .