A few questions:

1) How did we come up with the bivariate normal equation? Can anyone show me the derivation? (can't find on the net)

2) Are there any condition for 2 normal random variables to form a bivariate normal distro?
Can we just clobber up two rvs, and input the appropriate values in the variance-covariance matrix. If they are independent, then the covariance terms go to zero.

But I also read that you have to satisfy:
Z = aX + bY ~ N
before X and Y can be bivariate. Am not sure on this one.