點用尺規 畫出300度角?

Step:
1. Draw a line and mark 2 points A and B.
2. Using A as center and AB as radius draw an arc. Do the same for using B as center and AB as radius draw an arc.
3. The 2 arcs intersect at a point C, angle CAB will be = 60 degree, so the reflex angle CAB = 360 - 60 = 300 degree.
As far as I know, we cannot tri- sect an angle using only pen and ruler except for some special angles : 90, 60, 120, 180, 45 etc.