The perpendicular from the centre of a circle to a chord ........... the chord.

In the figure,if $OA = 5 \text{ cm}$,$AB = 8 \text{ cm}$,and $OD$ is perpendicular to $AB$,then $CD$ is equal to (in $\text{cm}$):

$A$ piece of a circle between two points on a circle is called ........ of the circle.

$ABCD$ is a cyclic quadrilateral. If $AB \parallel CD$,then prove that $BC = AD$.

In the figure,if $\angle ABC = 20^{\circ},$ then $\angle AOC$ is equal to (in $^{\circ}$)

If circles are drawn taking two sides of a triangle as diameters,prove that the point of intersection of these circles lies on the third side.