Diagram, then text:
The circle has radius 1 and centre at the origin.
The line is x = a
Now 1/a is the inverse of a, so a * 1/a = 1, and is fixed.
The line z from (1/a,0) to (x,y) is orthogonal to the radial line, so r/z = Y/a and the two triangles are similar
and r/(1/a) = a/R
Hence rR = a * (1/a) = 1, and both conclusions are true.
The point (x,y) follows a circular path, and rR = 1