Chapter 4 Activity 1
a
Angle ABC and angle ADC are supplementary angles since they add to 180 degrees. If you move the vertices of the quadrilateral or change the size of the circle, my observation still holds true. If you move D past C, angle ADC becomes congruent to angle ABC.
b
The exterior angle at B and the interior angle at D are congruent. My observation still holds true if I move the vertices of the quadrilateral or if I change the size of the circle.