The curved surface area of a cylinder is $3960\, cm^{2}$. If its radius is $21\, cm$,find its height in $cm$.

$A$ square sheet of paper,$44 \, cm$ long,is rolled along its length to form a cylinder. Find the volume of the cylinder so formed. (in $cm^3$)

In $\triangle ABC$,$\angle B = 90^{\circ}$,$AB = 28 \, cm$,and $BC = 21 \, cm$. If the triangle is revolved about the side $AB$,find the curved surface area of the cone so obtained. Similarly,if the triangle $ABC$ is revolved about the side $BC$,find the curved surface area of the cone so obtained.

Find the surface area of a sphere of radius $1.25 \, m$ $(\pi = 3.14)$ (in $m^2$).

The height of a cone is $9 \, cm$. The area of the base is $1386 \, cm^2$. Find its volume (in $cm^3$).

The radius of a sphere is increased by $10 \%$. Prove that the volume will be increased by $33.1 \%$ approximately.