AB $\perp$ BC, BD $\perp$ AC and CE bisects $\angle C$, $\angle A = 30^\circ$. Then what is $\angle CED$?
Step 1: Understanding the diagram In $\triangle ABC$, AB $\perp$ BC and $\angle A = 30^\circ$. So $\triangle ABC$ is right-angled at (b)
Step 2: Finding $\angle C$ Sum of angles in $\triangle ABC$: $\angle C = 180^\circ - 90^\circ - 30^\circ = 60^\circ$.
Step 3: Role of CE bisector CE is the angle bisector of $\angle C$, so $\angle ECD = \angle ECA = 30^\circ$.
Step 4: $\triangle CED$ analysis In right triangle $\triangle CED$, using geometry properties (and symmetry from perpendicular BD), $\angle CED$ = $60^\circ$.
In the adjoining figure, $\triangle CAB$ is a right triangle, right angled at A and $AD \perp BC$. Prove that $\triangle ADB \sim \triangle CDA$. Further, if $BC = 10$ cm and $CD = 2$ cm, find the length of AD. 
In the diagram, the lines QR and ST are parallel to each other. The shortest distance between these two lines is half the shortest distance between the point P and the line QR. What is the ratio of the area of the triangle PST to the area of the trapezium SQRT?
Note: The figure shown is representative

\( AB \) is a diameter of the circle. Compare:
Quantity A: The length of \( AB \)
Quantity B: The average (arithmetic mean) of the lengths of \( AC \) and \( AD \). 
O is the center of the circle above. 