(a) Show that if $y_1^2+y_2^2+y_3^2+y_4^2=1$, then the matrix
$$
Q=\left(\begin{array}{ccc}
y_1^2+y_2^2-y_3^2-y_4^2 & 2\left(y_2 y_3+y_1 y_4\right) & 2\left(y_2 y_4-y_1 y_3\right) \\
2\left(y_2 y_3-y_1 y_4\right) & y_1^2-y_2^2+y_3^2-y_4^2 & 2\left(y_3 y_4+y_1 y_2\right) \\
2\left(y_2 y_4+y_1 y_3\right) & 2\left(y_3 y_4-y_1 y_2\right) & y_1^2-y_2^2-y_3^2+y_4^2
\end{array}\right)
$$
is a proper orthogonal matrix. The numbers $y_1, y_2, y_3, y_4$ are known as Cayley-Klein parameters. (b) Write down a formula for $Q^{-1}$. (c) Prove the formulas
$$
y_1=\cos \frac{\varphi+\psi}{2} \cos \frac{\theta}{2}, y_2=\cos \frac{\varphi-\psi}{2} \sin \frac{\theta}{2}, y_3=\sin \frac{\varphi-\psi}{2} \sin \frac{\theta}{2}, y_4=\sin \frac{\varphi+\psi}{2} \cos \frac{\theta}{2} \text {, }
$$
relating the Cayley-Klein parameters and the Euler angles of Exercise 4.3.4, cf. [31; $\$ 4-5]$.