Geometric Proof - Dolfanescobar's Weblog | Middle … Proof Going Deeper ... definitions to prove an important theorem about angles. ... showing that the sum of the angle measures
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Geometric ProofGoing DeeperEssential question: How can you organize the deductive reasoning of a geometric proof?
You will use the Angle Addition Postulate and the following definitions to prove an important theorem about angles.
Opposite rays are two rays that have a common endpoint and form a straight line. A linear pair of angles is a pair of adjacent angles whose noncommon sides are opposite rays.
In the figure, __
› JK and
__ › JL are opposite rays; ∠MJK and ∠MJL are a linear
pair of angles.
Recall that two angles are complementary if the sum of their measures is 90°. Two angles are supplementary if the sum of their measures is 180°. The following theorem ties together some of the preceding ideas.
2-6
Linear Pair Theorem
If two angles form a linear pair, then they are supplementary.
Given: ∠MJK and ∠MJL are a linear pair of angles.
Prove: ∠MJK and ∠MJL are supplementary.
A Develop a plan for the proof.
Since it is given that ∠MJK and ∠MJL are a linear pair of angles, __
› JL and
__ › JK are opposite
rays. They form a straight angle. Explain why m∠MJK + m∠MJL must equal 180°.
B Complete the proof by writing the missing reasons. Choose from the following reasons.
p r o o f1
Statements Reasons
1. ∠MJK and ∠MJL are a linear pair. 1.
2. __
› JL and
__ › JK are opposite rays. 2. Definition of linear pair
3. __
› JL and
__ › JK form a straight line. 3.
4. m∠LJK = 180° 4. Definition of straight angle
5. m∠MJK + m∠MJL = m∠LJK 5.
6. m∠MJK + m∠MJL = 180° 6.
7. ∠MJK and ∠MJL are supplementary. 7. Definition of supplementary angles
Angle Addition Postulate Definition of opposite rays
1a. Is it possible to prove the theorem by measuring ∠MJK and ∠MJL in the figure and showing that the sum of the angle measures is 180°? Explain.
1b. The proof shows that if two angles form a linear pair, then they are supplementary. Is this statement true in the other direction? That is, if two angles are supplementary, must they be a linear pair? Why or why not?
Statements Reasons
1. ∠VXW and ∠ZXY are vertical angles. 1.
2. ∠VXW and ∠ZXY are formed by intersecting lines.
2. Definition of vertical angles
3. ∠VXW and ∠WXZ are a linear pair.∠WXZ and ∠ZXY are a linear pair.
1. Refer to the diagram of the stained-glass window and use the given plan to write a two-column proof.
Given: ∠1 and ∠3 are supplementary. ∠2 and ∠4 are supplementary. ∠3 ≅ ∠4
Prove: ∠1 ≅ ∠2 Plan: Use the definition of supplementary angles to write
the given information in terms of angle measures. Then use the Substitution Property of Equality and the Subtraction Property of Equality to conclude that ∠1 ≅ ∠2.
The position of a sprinter at the starting blocks is shown in the diagram. Which statement can be proved using the given information? Choose the best answer. 2. Given: ∠1 and ∠4 are right angles.
A ∠3 ≅ ∠5 C m∠1 + m∠4 = 90° B ∠1 ≅ ∠4 D m∠3 + m∠5 = 180°
3. Given: ∠2 and ∠3 are supplementary. ∠2 and ∠5 are supplementary. F ∠3 ≅ ∠5 H ∠3 and ∠5 are complementary. G ∠2 ≅ ∠5 J ∠1 and ∠2 are supplementary.