Parallelogram: Theorem 2

Interact with the applet below for a few minutes. Then, answer the questions that follow. Feel free to move the BIG WHITE POINTS anywhere you'd like! You can also adjust the size of the pink angle by using the slider.


What special type of quadrilateral was formed in the first half of your sliding-the-slider? How do you know this?


What else can you conclude about this special type of parallelogram? Be specific!


In the special quadrilateral above, suppose the pink angle measures 110 degrees. What would the measure of the blue angle be? What would the measure of each interior angle of this special quadrilateral be?


Write a 2-column or paragraph proof of what you've informally observed here. (Hint: This proof will involve a pair of congruent triangles!)

Quick Demo: 0:33 sec to 1:16 sec (BGM: Andy Hunter)