In the figure below, $CD$ is a chord of the semi-circle with centre $O.$

$OA$ is the radius of the circle. If $CD=10$ cm, $AB=2$ cm and $\overline{OA}\perp \overline{CD}$, then the length of $OB$ is _____.

In the figure below (not to scale), $PQ=PR=8$ cm and $O$ is the centre of the circle. If each of the chords $PQ$ and $PR$ makes an angle of $120°$ at the centre of the circle, then find the area of the shaded region.

In the above figure, $\overline{AB}$ and $\overline{FE}$ are intersected by a transversal, such that $\angle BGK=\angle EHL.$ Then $\overline{AB}$ is _____ to $\overline{EF.}$

In the above figure (not to scale), $l\parallel m, l_1\parallel m_1 \text{and} p\parallel q$ such that $\angle 1=90°, \angle 2=130° \text{and} \angle 3=70°.$ $\angle 4=$_____ degrees.

In the above figure (not to scale), $AB\parallel DE$ and $EC\parallel GF$. If $\angle EGF=100°$ and $\angle ECF=40°$, then find:

$\angle ABC=$_____ degrees.

In the above figure (not to scale), $AB\parallel DE$ and $EC\parallel GF$. If $\angle EGF=100°$ and $\angle ECF=40°$, then find:

$\angle GFD=$_____ degrees.

In the above figure (not to scale), $AB\parallel DE$ and $EC\parallel GF$. If $\angle EGF=100°$ and $\angle ECF=40°$, then find:

$\angle GDF=$_____ degrees.

The letters of the English alphabet which have only one line of symmetry are _____.

The angle whose supplement is three times its compliment is _____ degrees.