Langley Problem

Arthur Lee
Adjust the blue angles at A and B. Which combinations make it easy / difficult to solve for angle D?
This is a famous problem which looks simple but is in fact extremely difficult to solve with angle chasing. You may have a try, finding the unknown angle D in the figure.  The task here is not about directly solving this problem. You may consider varying the blue angles to other multiples of 10 degrees. When would it be easy or difficult for finding the angle D? When doing so, can you get a sense of what makes this problem difficult or easy to solve? You may use the applet to quickly adjust the blue angles and see how the figure changes. Do you have other suggestions of modifying this problem? More info about this problem can be found in these slides: