(2-8)  Just using the simple relationship that distance = speed x time, we get:

①  xC =  utC

②  r  =  ctC      (c being the speed of light)

③  y  =  ctN 

④  Using Pythagoras’ theorem:  

r2  =  xC2  +  y2

So:   c2tC2  =  u2tC2  +  c2tN2

Divide by c2:     tC2  =  (u2/ c2)tC2  +  tN2

Rearrange:   tC2(1 - u2/ c2)   =  tN2