Why is the red part true?
From: http://graphics.stanford.edu/courses/cs348a-17-winter/Papers/quaternion.pdf
HINT. Notice $\sin ^2\theta+\cos^2\theta=1$. What does $\theta\in [0,\pi]$ lead to? Consider its graph $(x=\cos \theta, y=\sin \theta )$ on the plane, with $x^2+y^2=1$, which is exactly an upper semicircle.
Copyright © 2021 JogjaFile Inc.
HINT. Notice $\sin ^2\theta+\cos^2\theta=1$. What does $\theta\in [0,\pi]$ lead to? Consider its graph $(x=\cos \theta, y=\sin \theta )$ on the plane, with $x^2+y^2=1$, which is exactly an upper semicircle.