This trigonometry textbook is different from other trigonometry books in that it is free to download. The reader is expected to do more than read the book and study the material by working out examples rather than just reading about them. So this book is not just about mathematical content but is also about the process of learning and doing mathematics. This book is designed not to be casually read but rather to be engaged.
Since this can be a difficult task, there are several features of the book designed to assist students in this endeavor. In particular, most sections of the book start with a beginning activity that review prior mathematical work that is necessary for the new section or introduce new concepts and definitions that will be used later in that section. Each section also contains several progress checks that are short exercises or activities designed to help readers determine if they are understanding the material. In addition, the text contains links to several interactive Geogebra applets or worksheets. These applets are usually part of a beginning activity or a progress check and are intended to be used as part of the textbook.