http://www.youtube.com/user/khanacademy#p/p

The above link will take you to Kahn's free videos on various subjects, including several topics in math. Scroll through the list to find what you want.
I was told that if I went to www.kahnacademy.org that I would be able to access his free videos. When I went to that web address, it indeed said kahnacaedmy, however I saw no link to his free videos or anything to do with kahnacademy. Instead the page was filled with 'related searches' that directed you to education sites that wanted money for information. My browser said that it had blocked popups from this site so I enabled popups which only brought up a WalMart advertisement. So I Googled kahnacademy and found the above site on youtube that contained his free videos. You may have to copy the link and paste it in your address bar to access the free site...Good Luck.
Two videos on similar triangles can be found by clicking on geometry. I tried copying the above link and pasting it my address bar...and it worked...it took me to his free videos on youtube. You can also right-click on the address above and click on 'open link'.