# Which angle can't be trisected?

- Author:
- Ku, Yin Bon (Albert)

To show that an angle cannot be trisected, it suffices to show that is not a constructible angle.
Let's consider . Then . Now we use the following trigonometric identity:
Let , we have
It can be shown that is irreducible over . Therefore, the degree of over is and hence by the Main Theorem, is not a constructible number.
In other words, is not a constructible angle, which in turn implies that cannot be trisected.