video 1 in lecture
Source: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CDMQtwIwAA&url=http%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DbCU3rcwnz8g&ei=NKk5UtSJH4bk9gTWkIGAAg&usg=AFQjCNFyy5qFo9Sz_0yp5hgYhobyHwJilg&sig2=QPaS578XRH1rQuxJhJH-gQ&bvm=bv.52288139,d.eWU
video 2 from lecture
Source: http://www.youtube.com/watch?v=23lo9vF9vvE&noredirect=1