Chapter 2

# Geometry Proof Starter Kit

IN THIS CHAPTER

**Introducing if-then logic**

**Theorizing about theorems and defining definitions**

**Understanding the complementary and supplementary angle theorems**

Traditional two-column geometry proofs are arguably the most important — and for many students, the most difficult — topic in a standard high school geometry course. In this chapter, I aim to declaw them! First, I give you a schematic drawing that shows you all the elements of a two-column proof and where they go. Then, I explain how you prove something using a deductive argument. After that, you’ll be ready for a starter kit of your first 18 theorems along with some proofs that illustrate how to use them. Before you know it, you’ll be a proof-writing crackerjack!

## The Lay of the (Proof) Land

A geometry proof involves a geometric diagram of some sort. You’re told some things that are true about the diagram (the *givens*), and you’re asked to prove that something else is true (the *prove* statement). Every proof proceeds as follows:

- You begin with one or more of the given facts about the diagram.
- You state something that follows from the given fact or facts; then you state something that ...

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