Point p is taken as PH vertical AD, vertical foot H, vertical foot PT AB and vertical foot T, then:

PH//AB，PT//AD，PH=AT=9，PT=AH=3，

MH/AM=PH/AN，MH=AM-AH=AM-3，

Let AM=y, then:

(y-3)/y=9/x，

y=3x/(x-9).

Is an algebraic expression of AM.

mn^2=am^2+an^2=[3x/(x-9)]^2+x^2

The area of the screen MNEF is s = Mn * ne = Mn * (16/9mn) =16/9 * Mn2.

= 16x^2/(x-9)^2+ 16/9*x^2

= 16x^2/9*[ 1/(x-9)^2+ 1]

Domain: 9