The binary-search algorithm takes log(n) time, because of the fact that the height of the tree (with n nodes) would be log(n).
How would you prove this?
Just look up the rigorous proof in Knuth, Volume 3 - Searching and Sorting Algorithms ... He does it far more rigorously than anyone else I can think of.
http://en.wikipedia.org/wiki/The_Art_of_Computer_Programming
You can find it in any good Computer Science library and on the bookshelves of many (very) old geeks.