Originally Posted by

**emakarov** Something I don't understand...

At this point we don't know if it is possible for the rotated image of C to lie on AB

**This we do know as |QD|=|DC| since triangles DBC and DQB have x, y and 40 degrees as shown.**

Then we check the condition required for P to be on AC.

This needs the angle "alpha" to be 60 degrees.

Then we examine the conditions for collinearity of A, Q, B.

*and* the image of B to lie on AC. Since above you defined Q but not P, I assume that we rotate BCD until C coincides with Q. The image of B, i.e., P, does not have to be on AC.

Right, so if we knew that P, D and C are collinear, i.e., that P lies on AC, then the proof would be finished.