Section 2.8 Proving Angle Relationships

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Worksheet – Section 2-8 Proving Angle Relationships
Objectives:
 Understand the Angle Addition Postulate and use it to find unknown angle measures
 Understand Supplements and Compliments and use to find unknown angle measures
 Use algebra to find unknown angle measure
 Use angle relation theorems to prove relationships with 2 column proofs
Angle Addition Postulate
R is in the interior of ∠PQS if and only if m∠PQR + m∠RQS = m∠PQS.
Example:
Find the measure of angle 1 if the measure of angle 2 is 56
degrees and
Practice:
If and , find the measure of angle 3. Justify each step.
Supplements and Complements

If two angles forma linear pair, then they are supplementary angle
If ∠1 and ∠2 form a linear pair, then m∠1 + m∠2 = 180.

If the noncommon sides of two adjacent angles form a right angle, then the angles are
complementary angles.
If ⊥ , then m∠3 + m∠4 = 90.
Example:
Suppose and form a linear pair. If and . Find x, , and .
Justify each step
Practice:
a. Find the measure of each numbered angle.
m∠7 = 5x + 5,
m∠8 = x – 5
b. Find the measure of each numbered angle.
m∠5 = 5x,
m∠6 = 4x + 6,
m∠7 = 10x,
m∠8 = 12x – 12
c. Find the measure of each numbered angle.
m∠11 = 11x,
m∠13 = 10x + 12
d. Find the measure of each numbered angle.
and are complimentary
and
Proving Angle Relationships
The following theorems hold true for angles and can be used in proofs dealing with angles
Congruent Supplements
Theorem
Angles supplement to the same angle or congruent angles are
congruent.
Congruent Compliments
Theorem
Angles compliment to the same angle or to congruent angles
are congruent.
Vertical Angles Theorem
If two angles are vertical angles, then they are congruent.
Theorem (Definition of
Perpendicular lines)
Perpendicular lines intersect to form four right angles.
Theorem (Definition of right
angles)
All right angles are congruent.
Theorem (Definition of
Perpendicular lines)
Perpendicular lines form congruent adjacent angles.
Example: Write a two-column proof.
Given: ∠ ABC and ∠CBD are complementary.
∠DBE and ∠CBD form a right angle.
Prove: ∠ ABC ≅ ∠DBE
Statements
Reasons
Example:
Complete each proof.
1. Given: ⊥ ;
∠1 and ∠3 are
complementary.
Prove: ∠2 ≅ ∠3
Proof:
Statements
Reasons
a. ⊥, ∠1 and ∠3 are complementary
a. ______________
b. _________________
b. Definition of ⊥
c. m∠ ABC = 90
c. Def. of right angle
d. m∠ ABC =m∠1 + m∠2
d. ______________
e. 90 = m∠1 + m∠2
e. Substitution
f. ∠1 and ∠2 are compliments
f. _______________
g. ∠2 ≅ ∠3
g.________________
Practice:
Given: ∠1 and ∠2 form a linear pair.
m∠1 + m∠3 = 180
Prove: ∠2 ≅ ∠3
Proof:
Statements
a. ∠1 and ∠2 form
a linear pair.
m∠1 + m∠3 = 180
Reasons
b. _________________
b. Def. of Linear Pair
c. ∠1 is suppl. to ∠3.
c. ________________
d. Congruent Supplements
d. _________________
a. Given
Homework Problems
Find the measure of each numbered angle and name the theorems that justify your work.
1. m∠2 = 57
2. m∠5 = 22
3. m∠1 = 38
4. m∠13 = 4x + 11,
m∠14 = 3x + 1
5. ∠9 and ∠10 are
complementary.
∠7 ≅ ∠9, m∠8 = 41
6. m∠2 = 4x – 26,
m∠3 = 3x + 4
7. Complete the following proof.
Given: ∠QPS ≅ ∠TPR
Prove: ∠QPR ≅ ∠TPS
Proof:
Statements
Reasons
7. Complete the following proof.
Given: bisects
Prove: ∠2 ≅ ∠3
Proof:
Statements
Reasons
×

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