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Find m BDC. SOLUTION. A B and ADC BCD , so by the Third Angles Theorem, ACD BDC . By the Triangle Sum Theorem, m ACD = 180° – 45° – 30° = 105°. So, m ACD = m BDC = 105° by the definition of congruent angles. ANSWER. EXAMPLE 4. Use the Third Angles Theorem. - PowerPoint PPT Presentation
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EXAMPLE 4 Use the Third Angles Theorem
Find m BDC.
So, m ACD = m BDC = 105° by the definition of congruent angles.
ANSWER
SOLUTION
A B and ADC BCD, so by the Third Angles Theorem, ACD BDC. By the Triangle Sum Theorem, m ACD = 180° – 45° – 30° = 105° .
EXAMPLE 5 Prove that triangles are congruent
Plan for Proof
AC AC.
a. Use the Reflexive Property to show that
b. Use the Third Angles Theorem to show that
B D
Write a proof.
GIVEN AD CB, DC AB
ACD CAB, CAD ACB
PROVE ACD CAB
EXAMPLE 5 Prove that triangles are congruent
Plan in Action
1. Given
2. Reflexive Property of Congruence
STATEMENTS REASONS
3. Given
4. Third Angles Theorem
1. AD CB, DC BA
2. a. AC AC.
3. ACD CAB,CAD ACB
4. b. B D
5. ACD CAB Definition of5.
GUIDED PRACTICE for Examples 4 and 5
SOLUTION
4. DCN.In the diagram, what is m
CDN NSR, DNC SNR then the third angles are also congruent NRS DCN = 75°
GUIDED PRACTICE for Examples 4 and 5
SOLUTION
(Proved from above sum)
By the definition of congruence, whatadditional information is needed toknow that
5.
NDC NSR.
CN NR, CDN NSR, DCN NRS
Given :
NDC NSR.Proved :
GUIDED PRACTICE for Examples 4 and 5
STATEMENT REASON
Given
Given
CDN NSR
DCN NRS
Therefore DC RS, DN SN as angles are congruent their sides are congruent.