Angle trisection with coefficients in the field generated by these numbers. Therefore, any number that is constructible by a sequence of steps is a root of a minimal polynomial whose degree is a power of two. Note also that radians (60 degrees, written 60°) is constructible. The argument below shows that it is impossible to construct a 20° angle. This implies that a 60° angle cannot be trisected, and thus that an arbitrary angle cannot be trisected. Denote the set of rational numbers by . If 60° could be trisected, the degree of a minimal polynomial of over would be a power
Angle bisector theorem of the length of side "AB" to the length of side "AC": and conversely, if a point "D" on the side "BC" of triangle "ABC" divides "BC" in the same ratio as the sides "AB" and "AC", then "AD" is the angle bisector of angle "∠ A". The generalized angle bisector theorem states that if "D" lies on the line "BC", then Angles "∠ BAD" and "∠ DAC" are equal. Therefore, the right hand sides of equations () and () are equal, so their left hand sides must also be equal. which is the angle bisector theorem. If angles "∠