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

Bisectors of angles $A, B$ and $C$ of a triangle $ABC$ intersect its circumcircle at $D, E$ and $F$ respectively. Prove that the angles of the triangle $DEF$ are $90^{\circ}-\frac{1}{2} A, 90^{\circ}-\frac{1}{2} B$ and $90^{\circ}-\frac{1}{2} C$.

The line segment joining the centre and any point on the circle is called a .......... of the circle.

$AB$ is a chord of a circle with centre $P$. Point $C$ is a point other than $A$ and $B$ on the major arc $AB$. If $\angle ACB = 50^{\circ}$,then find $\angle APB$. (in $^{\circ}$)

In an isosceles triangle $ABC$ with $AB = AC$,a circle passing through $B$ and $C$ intersects the sides $AB$ and $AC$ at $X$ and $Y$ respectively. Prove that $XY \parallel BC$.

Prove that equal chords of congruent circles subtend equal angles at their centres.