1. Postulates and Paragraph Proofs
LESSON 2–5
Postulates and Paragraph Proofs

2. Five-Minute Check (over Lesson 2–4) TEKS Then/Now New Vocabulary
Postulates: Points, Lines, and Planes
Key Concept: Intersections of Lines and Planes
Example 1: Real-World Example: Identifying Postulates
Example 2: Analyze Statements Using Postulates
Key Concept: The Proof Process
Example 3: Write a Paragraph Proof
Theorem 2.1: Midpoint Theorem
Lesson Menu

3. Determine whether the stated conclusion is valid based on the given information. If not, choose invalid. Given: A and B are supplementary. Conclusion: mA + mB = 180
A. valid
B. invalid
5-Minute Check 1

4. Determine whether the stated conclusion is valid based on the given information. If not, choose invalid. Given: Polygon RSTU has 4 sides. Conclusion: Polygon RSTU is a square.
A. valid
B. invalid
5-Minute Check 2

5. Determine whether the stated conclusion is valid based on the given information. If not, choose invalid. Given: A and B are congruent. Conclusion: ΔABC exists.
A. valid
B. invalid
5-Minute Check 3

6. Determine whether the stated conclusion is valid based on the given information. If not, choose invalid. Given: A and B are congruent. Conclusion: A and B are vertical angles.
A. valid
B. invalid
5-Minute Check 4

7. Determine whether the stated conclusion is valid based on the given information. If not, choose invalid. Given: mY in ΔWXY = 90. Conclusion: ΔWXY is a right triangle.
A. valid
B. invalid
5-Minute Check 5

8. How many noncollinear points define a plane?
B. 2
C. 3
D. 4
5-Minute Check 6

9. Mathematical Processes G.1(F), G.1(G)
Targeted TEKS
G.4(A) Distinguish between undefined terms, definitions, postulates, conjectures, and theorems.
Mathematical Processes
G.1(F), G.1(G)
TEKS

10. Identify and use basic postulates about points, lines, and planes.
You used deductive reasoning by applying the Law of Detachment and the Law of Syllogism.
Identify and use basic postulates about points, lines, and planes.
Write paragraph proofs.
Then/Now

11. postulate axiom proof theorem deductive argument paragraph proof
informal proof
Vocabulary

12. Concept

13. Concept

14. Identifying Postulates
ARCHITECTURE Explain how the picture illustrates that the statement is true. Then state the postulate that can be used to show the statement is true.
A. Points F and G lie in plane Q and on line m. Line m lies entirely in plane Q.
Answer: Points F and G lie on line m, and the line lies in plane Q. Postulate 2.5, which states that if two points lie in a plane, the entire line containing the points lies in that plane, shows that this is true.
Example 1

15. B. Points A and C determine a line.
Identifying Postulates
ARCHITECTURE Explain how the picture illustrates that the statement is true. Then state the postulate that can be used to show the statement is true.
B. Points A and C determine a line.
Answer: Points A and C lie along an edge, the line that they determine. Postulate 2.1, which says through any two points there is exactly one line, shows that this is true.
Example 1

16. ARCHITECTURE Refer to the picture
ARCHITECTURE Refer to the picture. State the postulate that can be used to show the statement is true. A. Plane P contains points E, B, and G.
A. Through any two points there is exactly one line.
B. A line contains at least two points.
C. A plane contains at least three noncollinear points.
D. A plane contains at least two noncollinear points.
Example 1

17. ARCHITECTURE Refer to the picture
ARCHITECTURE Refer to the picture. State the postulate that can be used to show the statement is true. B. Line AB and line BC intersect at point B.
A. Through any two points there is exactly one line.
B. A line contains at least two points.
C. If two lines intersect, then their intersection is exactly one point.
D. If two planes intersect, then their intersection is a line.
Example 1

18. If plane T contains contains point G, then plane T contains point G.
Analyze Statements Using Postulates
A. Determine whether the following statement is always, sometimes, or never true. Explain.
If plane T contains contains point G, then plane T contains point G.
Answer: Always; Postulate 2.5 states that if two points lie in a plane, then the entire line containing those points lies in the plane.
Example 2

19. contains three noncollinear points.
Analyze Statements Using Postulates
B. Determine whether the following statement is always, sometimes, or never true. Explain.
contains three noncollinear points.
Answer: Never; noncollinear points do not lie on the same line by definition.
Example 2

20. A. Determine whether the statement is always, sometimes, or never true
A. Determine whether the statement is always, sometimes, or never true. Plane A and plane B intersect in exactly one point.
A. always
B. sometimes
C. never
Example 2

21. B. Determine whether the statement is always, sometimes, or never true
B. Determine whether the statement is always, sometimes, or never true. Point N lies in plane X and point R lies in plane Z. You can draw only one line that contains both points N and R.
A. always
B. sometimes
C. never
Example 2

22. Concept

23. Given: Prove: ACD is a plane.
Write a Paragraph Proof
Given:
Prove: ACD is a plane.
Proof: and must intersect at C because if two lines intersect, then their intersection is exactly one point. Point A is on and point D is on Points A, C, and D are not collinear. Therefore, ACD is a plane as it contains three points not on the same line.
Example 3

24. Example 3

25. Proof: ?
Example 3

26. A. Definition of midpoint B. Segment Addition Postulate
C. Definition of congruent segments
D. Substitution
Example 3

27. Concept

28. Postulates and Paragraph Proofs
LESSON 2–5
Postulates and Paragraph Proofs