I read a book about statistics and machine learning, and can't understand assertion that: let $P(y) \sim N(y|\mu^*, \Sigma^*)$, i.e. multivariate normal distribution $p(y_1|\mu, \Sigma)$, where $\mu_1 = \Sigma(\Sigma_{11}^{-1}\mu_1 - \Sigma_{12}^{-1}(y_2 - \mu_2))$ and $\Sigma = \Sigma_{11} - \Sigma_{12}\Sigma_{22}^{-1}\Sigma_{21}$.
These formulas look difficult and I can't understand how these formulas were obtained. Can anyone give me a hint, how can this be proved?