Part A. When we have two chords that intercept at one of the exterme points of a circle, like this:
Then the relationship between the minor arc and the interior angle of the chords is:
therefore, if we substitute according to the given circle we have:
Therefore, angle a is 85°.
Part B. The angle formed by a tangent and a chord of a circle is half the the arc that is formed:
We have that:
Now, we substitute:
Therefore, "b" is 105°.
Part C. Given the following configuration:
The following relationship holds:
Now, we substitute the values according to the given circle:
Now, we divide both sides by "c":
Therefore, the value of "c" is 13.5
Part D. In the following configuration:
The following relationship holds:
Now, we substitute the values:
Solving the operations:
therefore, the angle "d" is 80 degrees.