Is there any method to do it by hand quickly? i want to show the angle $72$ can be trisected by compass and ruler. so i need to find the way to calculate it... help please!
closed as off-topic by Shailesh, Leucippus, JMP, user91500, Claude Leibovici Jun 13 '16 at 4:58
This question appears to be off-topic. The users who voted to close gave this specific reason:
- "This question is missing context or other details: Please improve the question by providing additional context, which ideally includes your thoughts on the problem and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level." – Shailesh, Leucippus, JMP, user91500, Claude Leibovici
We know that $60^\circ$ can be constructed and that $72^\circ$ can be constructed. By bisecting the $72^\circ$, we get $36^\circ$ and by subtracting the $60^\circ$ by the $36^\circ$, we get $24^\circ$. Thus, $24^\circ$ is constructable and $72^\circ$ can be trisected.