Please watch the three video links below.
https://www.youtube.com/watch?v=wTYvMpVITg8
https://www.youtube.com/watch?v=Zdc6Ih-ROt8
http://www.youtube.com/watch?v=jv4gTxWqeBE&feature=relmfu
http://www.youtube.com/watch?v=jHBqcKW4Xk0
HW: Complete 1 - 4 on WS Related Rates