OTCQ What is the measure of one angle in a equilateral/equiangular triangle?
Aim 4-3 How do we prove theorems about angles (part 1)? GG 30, GG 32, GG 34
Objectives SWBAT to construct the form of a proof and SWBAT to conjecture about appropriate statements.
Starters: Theorem 4-1: If 2 angles are right angles, then they are congruent. Theorem 4-1: If 2 angles are straight angles, then they are congruent. How could we justify these statements in a proof?
Prove Theorem 4-2 If two angles are straight angles, then they are congruent. A ED CB F StatementsReasons Given ABC is a straight angle and DEF is a straight angle. Prove ABC DEF.
4-3 #17.Prove Theorem 4-2 If two angles are straight angles, then they are congruent. A ED CB F StatementsReasons 1.Given 2.Definition of straight angle. 3.Definition of straight angle. 4.Definition of congruent. QED Given ABC is a straight angle and DEF is a Straight angle. Prove ABC DEF. 1. ABC is a straight angle and DEF is a straight angle. 2.m ABC = 180 ○ 3.m DEF = 180 ○. Conclusion ABC DEF.
Complementary angles are two angles the sum of whose degree measures is 90 ○. Supplementary angles are two angles the sum of whose degree measures is 180 ○.
Theorem 4-3: If 2 angles are complements of the same angle then they are congruent. Why?
Theorem 4-3: If 2 angles are complements of the same angle then they are congruent. Why? Given m 1= 45 ○ m 2= 45 ○ m 3= 45 ○ 1 3 2
Theorem 4-3: If 2 angles are complements of the same angle then they are congruent. Why? Given m 1= 45 ○ m 2= 45 ○ m 3= 45 ○ m 1+ m 2= 90 ○, hence 2 is the complement of 1. m 1+ m 3= 90○, hence 3 is the complement of 1. Since 2 and 3 are each the complement of 1, then 2 and 3 must be congruent.
Theorem 4-4: If 2 angles are congruent then their complements are congruent. Why?
Theorem 4-4: If 2 angles are congruent then their complements are congruent. Why? Given m 1= 30 ○ m 2= 30 ○
Theorem 4-4: If 2 angles are congruent then their complements are congruent. Why? Given m 1= 30 ○ m 2= 30 ○ If 3 is complementary to 1, what is the degree measure of 3? If 4 is complementary to 2, what is the degree measure of 4?
Theorem 4-4: If 2 angles are congruent then their complements are congruent. Why? Given m 1= 30 ○ m 2= 30 ○ If 3 is complementary to 1, what is the degree measure of 3? (90 ○ - 30 ○ = 60 ○ ) If 4 is complementary to 2, what is the degree measure of 4?
Theorem 4-4: If 2 angles are congruent then their complements are congruent. Why? 3 4 Given m 1= 30 ○ m 2= 30 ○ If 3 is complementary to 1, what is the degree measure of 3? (90 ○ - 30 ○ = 60 ○ ) If 4 is complementary to 2, what is the degree measure of 4? (90 ○ - 30 ○ = 60 ○ )
Theorem 4-5: If 2 angles are supplements of the same angle then they are congruent. Why? Please try to draw 2 angles that are supplementary to the same angle.
Theorem 4-5: If 2 angles are supplements of the same angle then they are congruent. Given: ABC is a straight angle, we can say that ABE is a supplement to EBC. A E D B C
Theorem 4-5: If 2 angles are supplements of the same angle then they are congruent. Given: ABC is a straight angle, we can say that ABE is a supplement to EBC. A E D B C Next, given that DBE is a straight angle, we can say that DBC is a supplement to EBC.
Theorem 4-5: If 2 angles are supplements of the same angle then they are congruent. Given: ABC is a straight angle, we can say that ABE is a supplement to EBC. A E D B C Next, given that DBE is a straight angle, we can say that DBC is a supplement to EBC. Conclusion: ABE DBC
Theorem 4-5: If 2 angles are supplements of the same angle then they are congruent. Given: ABC is a straight angle, we can say that ABE is a supplement to EBC. A E D B C Next, given that DBE is a straight angle, we can say that DBC is a supplement to EBC. Conclusion: ABE DBC 65 ○ 115 ○
Theorem 4-6: If 2 angles are congruent then their supplements are congruent. Why?
Theorem 4-6: If 2 angles are congruent then their supplements are congruent. Given: ABC is a straight angle. DBE is a straight angle. ABE DBC A E D B C Conclusion: ABD EBC 65 ○ 115 ○
Theorem 4-6: If 2 angles are congruent then their supplements are congruent. Given: ABC is a straight angle. DBE is a straight angle. ABE DBC A E D B C Conclusion: ABD EBC 65 ○ 115 ○
Linear pair of angles: 2 adjacent angles whose sum is a straight angle. ABE and EBC are a linear pair of angles. The others? A E D B C 65 ○ 115 ○
Linear pair of angles: 2 adjacent angles whose sum is a straight angle. ABE and EBC are a linear pair of angles. The others? EBC and CBD. CBD and DBA. DBA and ABE. There should always be 4 pairs of linear pairs when 2 lines intersect. A E D B C 65 ○ 115 ○ Why 4 pairs of linear pairs?
Linear pair of angles: 2 adjacent angles whose sum is a straight angle. ABE and EBC are a linear pair of angles. The others? EBC and CBD. CBD and DBA. DBA and ABE. There should always be 4 pairs of linear pairs when 2 lines intersect. A E D B C 65 ○ 115 ○ Theorem 4-7: Linear pairs of angles are supplementary.
Theorem 4-8: If 2 lines intersect to form congruent adjacent angles, then they are perpendicular
Since m 1 + m 2 =180 ○ and 1 2, we may substitute to say m 1 + m 1 =180 ○ and then 2 m 1 =180 ○ and then 2 2 m 1 =90 ○ We can do the same for 2, 3 and
Vertical angles: 2 angles in which the sides of one angle are opposite rays to the sides of the second angle. Theorem 4-9. If two lines intersect, then the vertical angles are congruent. Vertical angles: EBC and ABD. ABE and DBC. There should always be 2 pairs of vertical angles pairs when 2 lines intersect. A E D B C 65 ○ 115 ○