Mensuration (Pyramid, Cones and Spheres) - Volume and Surface Area of Composite Solids

A precast concrete drainage channel is \(25\) cm long and has a uniform cross-section.
Its outer cross-section is an isosceles trapezium with parallel sides \(16\) cm and \(28\) cm, perpendicular height \(8\) cm and sloping sides of \(10\) cm each. A semicircular groove of diameter \(8\) cm runs along the entire length, opening at the top and at both ends.
The shaded region represents concrete.

1. Find the area of the concrete cross-section, giving your answer exactly in terms of \(\pi\).

   [2]

2. Calculate the volume of concrete needed to make one channel.

   [1]

3. Before fitting, the channel is sealed over its entire exposed surface, including the curved groove, the underside and both ends. Calculate the area sealed.

   [3]

Give your answers to (b) and (c) correct to \(1\) decimal place.