1. Properties of Equality and Proving Segment & Angle Relationships
Section 2-6, 2-7, 2-8

2. Postulate - A statement that describes the relationship between basic terms in Geometry. Postulates are accepted as true without proof.
Examples of some Postulates:
Through any 2 points there is exactly 1 line.
Through any 3 noncollinear points there is exactly 1 plane.
A line contains at least 2 points.
A plane contains at least 3 noncollinear points.

3. Theorem
A conjecture or statement that can be shown to be true. Used like a definition or postulate.
Midpoint Theorem - If M is the
midpoint of AB, then AM  to MB. A M B

4. Proof
A logical argument in which each statement is supported by a statement that is true (or accepted as true).
Supporting evidence in a proof (the reason you can make the statement) are usually postulates, theorems, properties, definitions or given information.

5. Properties of Equality
The Distributive Property states that
a(b + c) = ab + ac.
Remember!

6. Segment Addition Postulate
If B is between A and C,
then AB + BC = AC
Converse: If AB + BC = AC, then B is between A and C. A B C

7. Example: Solving an Equation in Geometry
Write a justification for each step.

8. Example: Solving an Algebraic Equation
Write a justification for each step.
3(x - 2) = 42

9. Example: Proving an Algebraic Conditional Statement Write a justification for each step. If 3(x - 5/3) = 1, then x=2
Statement
Reason

10. Angle Addition Postulate
If R is in the interior of PQS, then mPQR + mRQS = m PQS.
Converse:
If mPQR + mRQS = m PQS,
then R is in the interior of PQS P R Q S

11. Segment and Angle Congruence Theorems -
Congruence of Segments (or Angles) is Reflexive, Symmetric and Transitive

12. Example: Identifying Property of Equality and Congruence
Identify the property that justifies each statement.
A. QRS  QRS
B. m1 = m2 so m2 = m1
C. AB  CD and CD  EF, so AB  EF.
D. 32° = 32°
Reflex. Prop. of .
Symm. Prop. of =
Trans. Prop of 
Reflex. Prop. of =

13. Practice Complete each sentence.
1. If the measures of two angles are ? , then the angles are congruent.
2. If two angles form a ? , then they are supplementary.
3. If two angles are complementary to the same angle, then the two angles are ? .
equal
linear pair
congruent

14. Additional Angle Theorems that You Should Know
Supplement Theorem – If two angles form a linear pair, then they are supplementary angles.
Complement Theorem – If the noncommon sides of two adjacent angles form a right angle, then the angles are complementary angles.

15. Three More Angle Theorems that You Should Know
Angles supplementary to the same angle, or to congruent angles, are congruent.
Angles complementary to the same angle, or to congruent angles, are congruent.
Vertical Angles Theorem – If two angles are vertical angles, then they are congruent.

16. Right Angles Theorems that Are Important:
Perpendicular lines intersect to form four right angles.
All right angles are congruent.
Perpendicular lines form congruent adjacent angles.
If two angles are congruent and supplementary, then each is a right angle.
If two congruent angles form a linear pair, then they are right angles.