........ is a unit of area.

As shown in the diagram,$\overline{ OA }$ and $\overline{ OB }$ are two radii of $\odot( O , 21 \text{ cm} )$ perpendicular to each other. If $OD = 10 \text{ cm}$,find the area of the shaded region. (in $\text{cm}^2$)

The area of a sector formed by a $12\,cm$ long arc in a circle with radius $12\,cm$ is $\ldots \ldots \ldots \, cm^{2}$.

In a circle with radius $50 \, cm$,the area of the sector formed by a $20 \, cm$ long arc is $\ldots \ldots \ldots cm^2$.

The circumference of a circular ground is $352\, m$. Find the area of the ground (in $m^2$).

The length of the minute hand of a clock is $14 \, cm$. Find the area swept by the minute hand in $5 \, minutes$. [use $\pi = \frac{22}{7}$]