Two Free Books
In the past, I have provided weblinks to copies of old mathematics texts. Often they are the product of digitial restoration projects.
Free "current" mathematics texts are also available. However, the phrase "Caveat emptor" comes to mind, even though the purchase price is nothing more than a click of a link.
Two options of free math texts to explore are:
 Abraham Hillman & Gerald Alexanderson's Algebra Through Problem Solving focuses on Pascal's Triangle, Fibonacci numbers, sequences, math induction, binomial theorem, combinations, permutations, polynomial equations, determinants, and inequalities....plus has some interesting problems
 Abraham Hillman, Gerald Alexanderson, & Mervin Newton's Complex Numbers and Trigonometry first presents a geometric view of complex numbers, then uses that view when exploring trigonometry....plus has some interesting problems. But be aware that it focuses on the use of a HP calculator, not a TI graphing calculator
This texts are available as downloads in pdf format, or they can be used online. And always, browse and use but remember my "caveat emptor." If you find some good, useable things, then great. If not, then life goes on.
