(3) When A and B move on the positive half axis of X axis and the negative half axis of Y axis respectively, the included angle between PF and PA is α.

Then 0 ≤α < 45.

h=PF=PA？ Cosα= = root number 2a/2? Coase α

∫0≤α< 45∴ radical number 2/2 < cos α ≤ 1

∴ a/2 < h ≤ root number 2a/2