(a).

The distance between the ground and table is .

The length of tall screen is .

The distance between the ground and screen is .

Consider the angle is the angle measured from eye level to the bottom of the screen and is the angle measured from eye level to the top of the screen.

The viewing angle is .

Observe the figure :

The viewing angle .

The distance opposite is .

Therefore , hence .

And , hence .

Since .

Susbtitute and in the above expression.

The function expressing interms of distance  is .

(b).

Graph:

Graph of is :

Observe the graph

The maximum distane is where the viewing angle is .

(a).

(b).The maximum distane is where the viewing angle is .