Where are proofs of Descartes rule of signs found?

[symbolipoint]

A few of the (assumed to be good) textbooks on College Algebra discuss the use of Descartes Rule of Signs, and a test for upper and lower bounds for real zeros for polynomial functions; but these ideas are never proved in the books that I found. In which college course, and in which Mathematics textbooks are proofs for these ideas found?

 

[Gib Z]

Only rigorous proof i found, it goes for 7 pages.

http://homepage.smc.edu/kennedy_john/POLYTHEOREMS.PDF