Adding up thin discs

You will write three small functions that find the volume of a solid of revolution by adding up thin discs and thin washers. The checker uses a cone, a sphere and a washer.

The idea. A thin disc of radius $r$ and thickness $t$ has volume $\pi r^2 t$. Cut the interval from $a$ to $b$ into $n$ equal slices. Measure the radius at the middle of each slice, and add the disc volumes. A washer has an outer and an inner radius, and its volume is the difference of two discs.

Your tasks.

1. Finish disc_volume(radius, thickness). Return the volume of one thin disc.
2. Finish solid_volume(f, a, b, n). Add up n discs, taking the radius f(x) at the middle of each slice.
3. Finish washer_volume(outer, inner, a, b, n). Add up n washers with the radii outer(x) and inner(x).