Because of the * * line of a, p and m, it is assumed that the angle between the direct AP and the X axis is α.

Then |AP|=|X0+ 1|÷cosα.

|PM|=|X0-3|÷cosα

So | AP | ⊙| PM | = (| x0+1| ⊙ cos α) ⊙ (| x0-3 | ⊙ cos α) = | x0+1| | x0-3.