Question

CF is a straight line.

D is the vertex of angle CDA and angle ADE
E is the vertex of angle DEB and angle BEF
(Look at the picture to be sure you understand all the statements above.)

Move points A and B to see how they affect the measures of the angles. What do you notice?

Answer 1

Explanation:

When you move point A horizontally to the right or left, the measures of angles ADC and ADE change, but the sum of their measures is always 180° since angles ADC and ADE are a linear pair and are supplementary, so the sum of their measures is 180°.

The same is true of point B moving horizontally and the measures of angles BEF and BED.