Lines and Symmetry — Answer Key
Part A: Fix the Sentence
Each sentence has an error. Rewrite it correctly on the line.
1. Fix the sentence:
Parallel lines cross each other at a right angle.
Corrected: Perpendicular lines cross each other at a right angle. Parallel lines never cross.
The corrected sentence is: "Perpendicular lines cross each other at a right angle. Parallel lines never cross.". The original sentence "Parallel lines cross each other at a right angle." contained an error that needed to be fixed.
2. Fix the sentence:
A ray has two endpoints and goes on forever in both directions.
Corrected: A ray has one endpoint and goes on forever in one direction.
The corrected sentence is: "A ray has one endpoint and goes on forever in one direction.". The original sentence "A ray has two endpoints and goes on forever in both directions." contained an error that needed to be fixed.
3. Fix the sentence:
Perpendicular lines form a 180° angle where they meet.
Corrected: Perpendicular lines form a 90° angle where they meet.
The corrected sentence is: "Perpendicular lines form a 90° angle where they meet.". The original sentence "Perpendicular lines form a 180° angle where they meet." contained an error that needed to be fixed.
Part B: Fill in the Blank
Write the missing word or number on each line.
1. Lines that never meet and stay the same distance apart are called parallel lines.
Parallel lines run in the same direction, are always the same distance apart, and never intersect.
2. A line segment has two endpoints.
A line segment is a part of a line bounded by two endpoints — it has a definite start and end.
3. Perpendicular lines form right angles where they cross.
When two lines are perpendicular they intersect at exactly 90° — a right angle.
4. A line goes on forever in both directions.
Unlike a ray (one direction) or a segment (finite), a line extends without end in both directions.
Part C: Short Answer
Answer each question in one or two complete sentences.
1. What is the difference between a line and a ray?
Sample answer: A line goes on forever in both directions with no endpoints, while a ray has one endpoint and goes on forever in only one direction.
Key distinction: lines are infinite in both directions; rays have a starting point and are infinite in one direction only.
2. Give an example of parallel lines you see in everyday life.
Sample answer: The two rails of a railroad track are parallel lines because they run side by side and never cross.
Any real-world example showing two lines that are always the same distance apart and never meet earns credit.