For each of the problems, twice the angle formed by the chords is equal to half the sum of the angles of the arcs.
So, for the first problem, we have

, so

.
For the second,

, so

.
For the last problem,

, so

.
Feel free to comment below if you have any questions!
Answer:
ABCD is a parallelogram.
Step-by-step explanation:
A parallelogram is a quadrilateral that has two parallel and equal pairs of opposite sides.
From the given diagram,
Given: AD = BC and AD || BC, then:
i. AB = DC (both pairs of opposite sides of a parallelogram are congruent)
ii. <ADC = < BCD and < DAB = < CBA
thus, AD || BC and AB || DC (both pairs of opposite sides of a parallelogram are parallel)
iii. < BAC = < DCA (alternate angle property)
iv. Join BD, line AC and BC are the diagonals of the quadrilateral which bisect each other. The two diagonals are at a right angle to each other.
v. <ADC + < BCD + < DAB + < CBA =
(sum of angles in a quadrilateral equals 4 right angles)
Therefore, ABCD is a parallelogram.