8-1

$\displaystyle \begin{pmatrix}1 & -\cos\delta \\ -\cos\delta & 1 \end{pmatrix}$

(20)

$\displaystyle \begin{vmatrix}1-\lambda & -\cos\delta \\ -\cos\delta & 1-\lambda \end{vmatrix} =$	$\displaystyle \left(1-\lambda\right)^2 -\cos^2\delta =\left[ \left(1-\lambda\right)+\cos\delta \right] \left[ \left(1-\lambda\right)-\cos\delta \right] =0$
	$% latex2html id marker 2358 $\displaystyle \quad \therefore \, \lambda = 1\pm \cos\delta \geq 0,\quad\because)\,-1\leq \cos\delta \leq +1$$