The cross-section of a tunnel is a rectangle of height h surmounted by a semicircular roof section of radius r. (See gure below). Suppose the cross-sectional area is A ftz. (a) Express the perimeter in terms of 1' only. The constant A will also appear in your answer. P(r) = (b) Find p '(r) = (c) Determine the radius of the cross-section which minimizes the perimeter. r= ft