I have a random vector $X = ( X_1 , X_2 )$ that has a bidimensional normal repartition with mean $0$ and covariation matrix :
$$ \Sigma = \left( \begin{array}{ccc} 1 & q \\ q & 1 \end{array} \right) $$
I need to calculate $q$, which is
$$ q = \mathrm{Corr}(X_1,X_2)$$
How can I calculate $\mathrm{Corr}(X_1,X_2)$ ?