I've rewritten the site into ASP.NET and moved many of the files. This file was merged with the following file,
http://tutorial.math.lamar.edu/Classes/CalcI/DerivativeProofs.aspx
You should be taken there shortly, however, if you aren't you may click on the link provided above.
If you arrived at this page by clicking on a link from somebody else's page you may want to let them know that the location of this page has changed so they can update their page.