For these exercices, I am asked to find the spectral measure of their processes if they are weakly stationary. However, I do not understand how to do so. https://i.stack.imgur.com/vUQIB.png
For exemple, for the 4th exercice,
$X_t = Z_1cos(ct) + Z_2 sin(ct)$
I find $\gamma(h) = \sigma^2 \cos(ch)$ Which is indeed the correct answer. However according to the solution, I am supposed to find the spectral measure being $\nu(d\lambda) = \frac{\sigma^2}{2} [\delta(d\lambda - c) + \delta(d\lambda + c)]$ using the reminders. How do I achieve this ?