To write proofs using geometric theorems 2.6 Prove Statements about Segments and Angles 2.7 Prove Angle Pair Relationships Objectives: To write proofs using geometric theorems To use and prove properties of special pairs of angles to find angle measurements

Thanks a lot, Euclid! Recall that it was the development of civilization in general and specifically a series of clever ancient Greeks who are to be thanked (or blamed) for the insistence on reason and proof in mathematics.

Premises in Geometric Arguments The following is a list of premises that can be used in geometric proofs: Definitions and undefined terms Properties of algebra, equality, and congruence Postulates of geometry Previously accepted or proven geometric conjectures (theorems)

Properties of Equality Maybe you remember these from Algebra. Reflexive Property of Equality For any real number a, a = a. Symmetric Property of Equality For any real numbers a and b, if a = b, then b = a. Transitive Property of Equality For any real numbers a, b, and c, if a = b and b = c, then a = c.

Theorems of Congruence Congruence of Segments Segment congruence is reflexive, symmetric, and transitive.

Theorems of Congruence Congruence of Angles Angle congruence is reflexive, symmetric, and transitive.

Example 1a Given: Prove: 2. has length AB Statements Reasons 1. 2.Ruler Postulate 3. AB = AB 3.Reflexive Prop. of = 4. 4.Definition of Congruent Segments

Example 1b Given: Prove:

Example 2 Prove the following: If M is the midpoint of AB, then AB is twice AM and AM is one half of AB. Given: M is the midpoint of Prove: AB = 2AM and AM = (1/2)AB

Example 3a If there was a right angle in Denton, TX, and other right angle in that place in Greece with all the ruins (Athens), what would be true about their measures?

Right Angle Congruence Theorem All right angles are congruent. Yes, it seems obvious, but can you prove it? What would be your Given information? What would you have to prove?

Example 3b Given: < A and < B are right angles Prove:

Linear Pair Postulate If two angles form a linear pair, then they are supplementary. Do we have to prove this?

Example 4 Given: Prove:

Congruent Supplements Suppose your angles were numbered as shown. Notice angles 1 and 2 are supplementary. Notice also that 2 and 3 are supplementary. What must be true about angles 1 and 3?

Congruent Supplement Theorem If two angles are supplementary to the same angle (or to congruent angles), then they are congruent.

Example 5 Prove the Congruent Supplement Theorem. Given: < 1 and < 2 are supplementary < 2 and < 3 are supplementary Prove:

What to Prove Notice that you can essentially have two kinds of proofs: Proof of the Theorem Someone has already proven this. You are just showing your peerless deductive skills to prove it, too. YOU CANNOT USE THE THEOREM TO PROVE THE THEOREM! Proof Using the Theorem (or Postulate)

Congruent Complement Theorem If two angles are complementary to the same angle (or to congruent angles), then they are congruent. You’ll have to prove this in your homework.

Vertical Angle Congruence Theorem Vertical angles are congruent.

Example 6 Prove the Vertical Angles Congruence Theorem. Given: < 1 and < 3 are vertical angles Prove:

Example 7 Given: Prove:

Example 8 Given: Prove: < 3 and < 4 are supplements

Example 9