Chapter 2 Reasoning and Proof 1Chapter 2: Reasoning and
Proof
Slide 2
Conditional Statements Chapter 2: Reasoning and Proof2 Lesson 2
1 Objectives 1 Recognize conditional statements 2 To write
converses of conditional statements
Slide 3
Conditional Statements Chapter 2: Reasoning and Proof3 Lesson 2
1 Key Concepts A conditional statement is _______________________.
Every conditional statement has two parts. The part following the
If is the ____________. The part following the then is the
__________.
Slide 4
Conditional Statements Chapter 2: Reasoning and Proof4 Lesson 2
1 Identify the hypothesis and the conclusion: If two lines are
parallel, then the lines are coplanar. Conclusion: Hypothesis:
Slide 5
Conditional Statements Chapter 2: Reasoning and Proof5 Lesson 2
1 Write the statement as a conditional: An acute angle measures
less than 90. If an angle is acute, then it measures less than 90.
The subject of the sentence is An acute angle. The hypothesis is An
angle is acute. The first part of the conditional is If an angle is
acute. The verb and object of the sentence are measures less than
90. The conclusion is It measures less than 90. The second part of
the conditional is then it measures less than 90.
Slide 6
Conditional Statements Chapter 2: Reasoning and Proof6 Lesson 2
1 Key Concepts A _________________ is a case in which the
hypothesis is true and the conclusion is false. To show that a
conditional is false, you need to find only one
counterexample.
Slide 7
Conditional Statements Chapter 2: Reasoning and Proof7 Lesson 2
1 Find a counterexample to show that this conditional is false: If
x 2 0, then x 0.
Slide 8
Conditional Statements Chapter 2: Reasoning and Proof8 Lesson 2
1 Use the Venn diagram below. What does it mean to be inside the
large circle but outside the small circle?
Slide 9
Conditional Statements Chapter 2: Reasoning and Proof9 Lesson 2
1 Key Concepts In the converse of a conditional statement the
hypothesis and conclusion are switched. Conditional:If p, then q
Converse:If q, then p
Slide 10
Conditional Statements Chapter 2: Reasoning and Proof10 Lesson
2 1 The Mad Hatter states: You might just as well say that I see
what I eat is the same thing as I eat what I see! Provide a
counterexample to show that one of the Mad Hatters statements is
false.
Slide 11
Conditional Statements Chapter 2: Reasoning and Proof11 Lesson
2 1 Write the converse of the conditional: If x = 9, then x + 3 =
12.
Slide 12
Conditional Statements Chapter 2: Reasoning and Proof12 Lesson
2 1 Write the converse of the conditional, and determine the truth
value of each: If a 2 = 25, a = 5.
Slide 13
Conditional Statements Chapter 2: Reasoning and Proof13 Lesson
2 1 Lesson Quiz Use the following conditional for Exercises 13. If
a circles radius is 2 m, then its diameter is 4 m. 1.Identify the
hypothesis and conclusion. 2.Write the converse. If a circles
diameter is 4 m, then its radius is 2 m. 3.Determine the truth
value of the conditional and its converse. Both are true. Show that
each conditional is false by finding a counterexample. 4.If lines
do not intersect, then they are parallel. skew lines 5.All numbers
containing the digit 0 are divisible by 10. Sample: 105
Biconditionals and Definitions Chapter 2: Reasoning and Proof15
Lesson 2 2 Objectives 1 To write biconditionals 2 To recognize good
definitions
Slide 16
Biconditionals and Definitions Chapter 2: Reasoning and Proof16
Lesson 2 2 Key Concepts If a conditional and its converse are both
true, the statement is said to be ________________. Biconditional
statements are often stated in the form if and only if IFF short
for if and only if - symbol for if and only if An angle is a right
angle if and only if it measures 90.
Slide 17
Biconditionals and Definitions Chapter 2: Reasoning and Proof17
Lesson 2 2 Consider this true conditional statement. Write its
converse. If the converse is also true, combine the statements as a
biconditional.
Slide 18
Biconditionals and Definitions Chapter 2: Reasoning and Proof18
Lesson 2 2 Write the two statements that form this biconditional.
Conditional: Converse: Biconditional: Lines are skew if and only if
they are noncoplanar.
Slide 19
Biconditionals and Definitions Chapter 2: Reasoning and Proof19
Lesson 2 2 Key Concepts A good definition is reversible. That means
that you can write a good definition as a true biconditional. The
Reversibility Test The reverse (converse) of a definition must be
true. If the reverse of a statement is false, then the statement is
not a good definition.
Slide 20
Biconditionals and Definitions Chapter 2: Reasoning and Proof20
Lesson 2 2 Show that this definition of triangle is reversible.
Then write it as a true biconditional. Definition: A triangle is a
polygon with exactly three sides.
Slide 21
Biconditionals and Definitions Chapter 2: Reasoning and Proof21
Lesson 2 2 Is the following statement a good definition? Explain.
An apple is a fruit that contains seeds.
Slide 22
Biconditionals and Definitions Chapter 2: Reasoning and Proof22
Lesson 2 2 Lesson Quiz 1.Write the converse of the statement. If it
rains, then the car gets wet. 2.Write the statement above and its
converse as a biconditional. 3.Write the two conditional statements
that make up the biconditional. An angle is a straight angle if and
only if it measures 180. Is each statement a good definition? If
not, find a counterexample. 4.The midpoint of a line segment is the
point that divides the segment into two congruent segments. 5.A
line segment is a part of a line.
Slide 23
Biconditionals and Definitions Chapter 2: Reasoning and Proof23
Lesson 2 2 Homework Page 90 2-26 even
Slide 24
Deductive Reasoning Chapter 2: Reasoning and Proof24 Lesson 2 3
Objectives 1 To use the Law of Detachment 2 To use the Law of
Syllogism
Slide 25
Deductive Reasoning Chapter 2: Reasoning and Proof25 Lesson 2 3
Key Concepts Deductive Reasoning (or logical reasoning) is If the
given statements are true, deductive reasoning produces a true
conclusion.
Slide 26
Deductive Reasoning Chapter 2: Reasoning and Proof26 Lesson 2 3
Key Concepts Law of Detachment If a conditional is true and its
hypothesis is true, then its conclusion is true. In symbolic form:
If p q is a true statement and p is true, then q is true.
Slide 27
Deductive Reasoning Chapter 2: Reasoning and Proof27 Lesson 2 3
A gardener knows that if it rains, the garden will be watered. It
is raining. What conclusion can he make?
Slide 28
Deductive Reasoning Chapter 2: Reasoning and Proof28 Lesson 2 3
For the given statements, what can you conclude? Given: If an angle
acute, then its measure is less than 90. A is acute.
Slide 29
Deductive Reasoning Chapter 2: Reasoning and Proof29 Lesson 2 3
Does the following argument illustrate the Law of Detachment?
Given: If you make a field goal in basketball, you score two
points. Jenna scored two points in basketball.
Slide 30
Deductive Reasoning Chapter 2: Reasoning and Proof30 Lesson 2 3
Key Concepts Law of Syllogism If p q and q r are true statements,
then p r is a true statement.
Slide 31
Deductive Reasoning Chapter 2: Reasoning and Proof31 Lesson 2 3
Use the Law of Syllogism to draw a conclusion from the following
true statements: If a quadrilateral is a square, then it contains
four right angles. If a quadrilateral contains four right angles,
then it is a rectangle.
Slide 32
Deductive Reasoning Chapter 2: Reasoning and Proof32 Lesson 2 3
Use the Laws of Detachment and Syllogism to draw a possible
conclusion. If the circus is in town, then there are tents at the
fairground. If there are tents at the fairground, then Paul is
working as a night watchman. The circus is in town.
Slide 33
Deductive Reasoning Chapter 2: Reasoning and Proof33 Lesson 2 3
Lesson Quiz Use the three statements below. A. If games are
canceled, then Maria reads a book. B. If it snows, then games are
canceled. C. It is snowing. 1.Using only statements A and B, what
can you conclude? 2.Using only statements B and C, what can you
conclude? 3.Using statements A, B, and C, what can you conclude?
4.Suppose both statement B and games are canceled are true. Can you
conclude that statement C is true? Explain.
Reasoning in Algebra Chapter 2: Reasoning and Proof35 Lesson 2
4 Objectives 1 To connect reasoning in algebra and geometry
Slide 36
Addition Property If a = b, then a + c = b + c. Subtraction
Property If a = b, then a c = b c. Multiplication Property If a =
b, then a c = b c. Division Property If a = b and c 0, then
Reasoning in Algebra Chapter 2: Reasoning and Proof36 Lesson 2 4
Key Concepts Properties of Equality
Slide 37
Reasoning in Algebra Chapter 2: Reasoning and Proof37 Lesson 2
4 Key Concepts Properties of Equality continued Reflexive Property
a = a Symmetric Property If a = b, then b = a. Transitive Property
If a = b and b = c, then a = c. Substitution Property If a = b,
then b can replace a in any expression.
Slide 38
The Distributive Property a(b + c) = ab + ac Reasoning in
Algebra Chapter 2: Reasoning and Proof38 Lesson 2 4 Key
Concepts
Slide 39
Reasoning in Algebra Chapter 2: Reasoning and Proof39 Lesson 2
4 Justify each step used to solve 5x 12 = 32 + x for x. 1.5x = 44 +
x 2.4x = 44 3.x = 11 Given: 5x 12 = 32 + x
Slide 40
Reasoning in Algebra Chapter 2: Reasoning and Proof40 Lesson 2
4 Suppose that points A, B, and C are collinear with point B
between points A and C. Solve for x if AC = 21, BC = 15 x, and AB =
4 + 2x. Justify each step. AB + BC=AC (4 + 2x) + (15 x)=21 19 +
x=21 x=2x=2
Slide 41
Reasoning in Algebra Chapter 2: Reasoning and Proof41 Lesson 2
4 Key Concepts Properties of Congruence
Slide 42
Reasoning in Algebra Chapter 2: Reasoning and Proof42 Lesson 2
4 Name the property that justifies each statement. a.If x = y and y
+ 4 = 3x, then x + 4 = 3x. b.If x + 4 = 3x, then 4 = 2x.
Slide 43
Reasoning in Algebra Chapter 2: Reasoning and Proof43 Lesson 2
4 (continued) c.If P Q, Q R, and R S, then P S
Slide 44
Lesson Quiz Reasoning in Algebra Chapter 2: Reasoning and
Proof44 Lesson 2 4 Name the justification for each statement. 1.ab
= ab 2.If m ABC + 40 = 85, then m ABC = 45. 3.If k = m and k + w =
12, then m + w = 12. 4.If B is a point in the interior of AOC, then
m AOB + m BOC = m AOC. 5.Fill in the missing information. Given: AC
= 36 a.AB + BC = ACi. ? b.3x + 2x + 1 = 36ii. ? c. ? iii. ? d.5x =
35iv. ? e.x = ? v. ?
Slide 45
Reasoning in Algebra Chapter 2: Reasoning and Proof45 Lesson 2
4 Homework Pages 105 107 2 22 even, 28, 31
Slide 46
Proving Angles Congruent Chapter 2: Reasoning and Proof46
Lesson 2 5 Objectives 1 To prove and apply theorems about
angles
Slide 47
Proving Angles Congruent Chapter 2: Reasoning and Proof47
Lesson 2 5 Key Concepts A statement that you prove true is a
____________. A paragraph proof is written as sentences in a
paragraph. A __________ is a convincing argument that uses
deductive reasoning. Given: lists what you know from the hypothesis
of the theorem Prove: the conclusion of the theorem Diagram:
records the given information visually
Slide 48
Proving Angles Congruent Chapter 2: Reasoning and Proof48
Lesson 2 5 Key Concepts Given: 1 and 2 are vertical angles Prove: 1
2 Proof: By the Angle Addition Postulate, m 1 + m 3 = 180 and m 2 +
m 3 = 180. By substitution, m 1 + m 3 = m 2 + m 3. Subtract m 3
from each side. You get m 1 = m 2, or 1 2. Theorem: Vertical angles
are congruent.
Slide 49
Proving Angles Congruent Chapter 2: Reasoning and Proof49
Lesson 2 5 Find the value of x.
Slide 50
Proving Angles Congruent Chapter 2: Reasoning and Proof50
Lesson 2 5 Key Concepts Given: 1 and 2 are supplementary 3 and 2
are supplementary Prove: 1 3 Proof: By the definition of
supplementary angles, m 1 + m 2 = 180 and m 3 + m 2 = 180. By
substitution, m 1 + m 2 = m 3 + m 2. Subtract m 2 from each side.
You get m 1 = m 3, or 1 3. Theorem: If two angles are supplements
of the same angle, then the two angles are congruent.
Slide 51
Proving Angles Congruent Chapter 2: Reasoning and Proof51
Lesson 2 5 Key Concepts Given: 1 and 2 are supplementary 3 and 4
are supplementary 2 4 Prove: 1 3 Proof: By the definition of
supplementary angles, m 1 + m 2 = 180 and m 3 + m 4 = 180. By
substitution, m 1 + m 2 = m 3 + m 4. Since 2 4, by the definition
of congruence m 2 = m 4. By substitution m 1 + m 4 = m 3 + m 4.
Subtract m 4 from each side. You get m 1 = m 3, or 1 3. Theorem: If
two angles are supplements of congruent angles, then the two angles
are congruent.
Slide 52
Proving Angles Congruent Chapter 2: Reasoning and Proof52
Lesson 2 5 Key Concepts Theorem: If two angles are complements of
the same angle (or of congruent angles), then the two angles are
congruent. Theorem: All right angles are congruent. Theorem: If two
angles are congruent and supplementary, then each is a right
angle.
Slide 53
Proving Angles Congruent Chapter 2: Reasoning and Proof53
Lesson 2 5 Write a paragraph proof using the given, what you are to
prove, and the diagram. Given: WX = YZ Prove: WY = XZ
Slide 54
Proving Angles Congruent Chapter 2: Reasoning and Proof54
Lesson 2 5 Lesson Quiz Use the diagram and m ABS = 3x + 6 and m RBC
= 5x 20 for Exercises 14. 1. Find x. 2. Find m ABS. 4. Without
using the Vertical Angle Theorem, what theorem can you use to prove
that ABR SBC? 3. Find m SBC.