I am trying to show that the with a Wiener Process $w(t)$, then $\mathbb{E}[|w(t_1)w(t_2)|] = (\frac{2a}{\pi}) \sqrt{t_1 \cdot t_2} (\cos \theta + \theta \sin \theta)$, given $\sin \theta = \sqrt{\frac{t_1}{t_2}}$.
So I know that $R_x(t_1,t_2) = a\cdot\min(t_1,t_2)) = \mathbb{E}[w(t_1)w(t_2)]$
I tried to take the approach that $|w(t)| = \sqrt{w(t)^2}$. But I still can't seem to solve it.