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Geometric Proof
Warm Up
Determine whether each statement is true or
false. If false, give a counterexample.
1. It two angles are complementary, then they are
not congruent. false; 45° and 45°
2. If two angles are congruent to the same angle,
then they are congruent to each other. true
3. Supplementary angles are congruent.
false; 60° and 120°
Holt McDougal Geometry
Geometric Proof
Essential Question
How do we write two-column proofs
and prove geometric theorems by using
deductive reasoning?
Unit 2Day 8
Geometric Proof
2-3
Holt McDougal Geometry
Geometric Proof
A geometric proof begins with Given and Prove
statements, which restate the hypothesis and
conclusion of the conjecture. In a two-column
proof, you list the steps of the proof in the left
column. You write the matching reason for each
step in the right column.
Holt McDougal Geometry
Geometric Proof
Example 1: Completing a Two-Column Proof
Fill in the blanks to complete the two-column
proof.
Given: XY
Prove: XY  XY
Statements
1.
XY
2. XY = XY
3. XY
.  XY
Holt McDougal Geometry
Reasons
1. Given
2. Reflex.
.
Prop. of =
3. Def. of  segs.
Geometric Proof
Example 2
Fill in the blanks to complete a two-column proof of one
case of the Congruent Supplements Theorem.
Given: 1 and 2 are supplementary, and
2 and 3 are supplementary.
Prove: 1  3
Proof:
a. 1 and 2 are supp., and
2 and 3 are supp.
b. m1 + m2 = m2 + m3
c. Subtr. Prop. of =
d. 1  3
Holt McDougal Geometry
Geometric Proof
Holt McDougal Geometry
Geometric Proof
Example 3: Writing a Two-Column Proof from a Plan
Use the given plan to write a two-column proof.
Given: 1 and 2 are supplementary, and
1  3
Prove: 3 and 2 are supplementary.
Plan: Use the definitions of supplementary and congruent angles
and substitution to show that m3 + m2 = 180°. By the
definition of supplementary angles, 3 and 2 are supplementary.
Holt McDougal Geometry
Geometric Proof
Example 3 Continued
Statements
1. 1 and 2 are supplementary.
Reasons
1. Given
1  3
2. m1 + m2 = 180°
of supp. s
2. Def.
.
= m3
3. m1
.
3. Def. of  s
4. m3 + m2 = 180°
4. Subst.
5. 3 and 2 are supplementary 5. Def. of supp. s
Holt McDougal Geometry
Geometric Proof
Example 4
Use the given plan to write a two-column proof if one
case of Congruent Complements Theorem.
Given: 1 and 2 are complementary, and
2 and 3 are complementary.
Prove: 1  3
Plan: The measures of complementary angles add to 90° by
definition. Use substitution to show that the sums of both pairs are
equal. Use the Subtraction Property and the definition of
congruent angles to conclude that 1  3.
Holt McDougal Geometry
Geometric Proof
Check It Out! Example 3 Continued
Statements
Reasons
1. 1 and 2 are complementary. 1. Given
2 and 3 are complementary.
2. m1 + m2 = 90°
m2 + m3 = 90°
of comp. s
2. Def.
.
+ m2 = m2 + m3 3. Subst.
3. m1
.
4. m2 = m2
4. Reflex. Prop. of =
5. m1 = m3
5. Subtr. Prop. of =
6. 1  3
6. Def. of  s
Holt McDougal Geometry
Geometric Proof
Example 5
Use the given plan to write a two-column proof.
Given: 1, 2 , 3, 4
Prove: m1 + m2 = m1 + m4
Plan: Use the linear Pair Theorem to show that the
angle pairs are supplementary. Then use the
definition of supplementary and substitution.
Statements
1. 1 and 2 are supp.
Reasons
1. Linear pair thm.
1 and 4 are supp.
2. m1 + m2 = 180°,
2. Def. of supp. s
m1 + m4 = 180°
3. m1 + m2 = m1 + m4
Holt McDougal Geometry
3. Subst.
Geometric Proof
Assignment:
• p. 61 # 1-4
• p. 62 # 6,7
Holt McDougal Geometry
Geometric Proof
Holt McDougal Geometry
Geometric Proof
Holt McDougal Geometry