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** Index**

Many of the results in this chapter are informal and geometrical, and do not follow
logically from our assumptions. I will freely use properties of similar triangles,
parallelograms, and trigonometric functions. Some of the results (e.g., those
involving trigonometric identities) will be rederived later in a more rigorous form.
Every statement labeled **Theorem** or **Definition** is part of our logical
development.

**Subsections**