A great circle is a circular ring on the sphere, the centre for which, coincides with the centre of the sphere. 

So the radius of the Great Circle is same as that for the sphere.

A small circle of a sphere, is a circle drawn on the sphere, with lesser radius than the sphere itself. 

Total surface area of a zone or frustum = 2 π R h +  π r₁²π r₂²

What is the volume of a spherical cap of height h units?

As derived here(3R - h)  πh²

Height of the geometric centroid above the centre of the sphere = (3 (2R - h) ²4 (3R - h) units

Volume of the hemisphere = (2/3) * π * Radius³  

Surface Area of the hemisphere = Curved surface area + area of base = 2 * π * Radius² π * Radius² = 3 π Radius²