What is the solid angle subtended by a hemisphere at its centre?

Solid angle subtended by hemisphere at centre is ...... .

The total plane angle subtended by a circle at its centre is

The total plane angle subtended by a circle at its centre is

Steradian is the solid angle subtended at the centre of the sphere by its surface whose area is equal to ____ of the sphere.

The chord of a circle is equal to its radius, find the angle subtended by this chord at the centre.