Check the picture below.
keeping in mind that the difference of the distances of d1 and d2 is always a consistent constant value, as you can see in the left-hand-side of the picture, if we keep on sliding on the hyperbolic line shrinking d2 from being slanted to be horizontal, once we put d1 and d2 on a horizontal line, as you see on the right-hand-side, if we subtract the pink line from the orange line, what's leftover is, yeap, you guessed it.
