Therefore: DC=AB=xm, DC‖AB.

Therefore: △EDC∽△ EAF.

Therefore: DC/AF=ED/AE.

AE = 30m，AF = 40m。

So: x/40 = (AE-ad)/AE.

Therefore: x/40 = (30-ad)/30.

Therefore: AD = 30-3/4x

(2)y=AB？ AD=x(30-3/4x)

So: y=-3/4(x-20)? +300

So when x takes 20, the value of y is the largest. The maximum value is 300.