And there are even more of his lectures on the MIT OCW website.
Oh, and since the topic was positive definite matrices, there are 3 ways to tell if you have one (t=47:17 on the video):
1. all the pivots are > 0
2. all the eigenvalues are > 0
3. \quad x^TKx > 0 \quad \forall x \neq 0