So when learning about quaternions from this video:
there is computation of cos of 110 degress angle, but I am not able to figure out what kind of theorem lead to given in video and screenshot below equation for cos or sin. I would grately appreciate any help. I can get h2 different way based on circle and right angles but cannot make it up with this 110 deg.
He is using the tangens half-angle formula, also called Weierstrass substitution