Lines and Symmetry — Answer Key
Part A: Fill in the Blank
Write the missing word or number on each line.
1. A line segment has two endpoints that mark where it starts and ends.
Unlike a line that goes on forever, a line segment stops at two specific points called endpoints, which mark where it begins and where it ends.
2. The vertical and horizontal lines of a grid are perpendicular to each other.
Grid lines meet at right angles (90 degrees), and lines that cross at right angles are called perpendicular. That is why graph paper corners are always square.
3. The letter B has 1 line(s) of symmetry.
The letter B has 1 line of symmetry along a horizontal line through its middle, because the top bump and bottom bump are mirror images of each other.
4. A right angle measures exactly 90 degrees.
A right angle is defined as exactly 90 degrees. You can spot one by looking for the small square symbol drawn in the corner where two lines meet.
5. A regular nonagon has 9 lines of symmetry.
A regular nonagon has 9 equal sides, and every regular polygon has the same number of symmetry lines as sides, so it has 9 lines of symmetry.
6. Lines in the same plane that never cross are parallel.
Parallel lines lie in the same flat surface and stay the same distance apart forever, so they never cross no matter how far they extend.
7. The letter C has 1 line(s) of symmetry.
The letter C has 1 line of symmetry running horizontally through its middle, because the top curve and bottom curve are mirror images of each other.
8. A diagonal of a square connects two opposite vertices.
A diagonal stretches across the inside of a shape from one corner (vertex) to the corner on the opposite side. In a square, each diagonal connects two vertices that do not share a side.
9. The point where two lines cross is called the point of intersection.
The exact spot where two lines cross each other is called the point of intersection. "Intersect" means to cut across or meet, so the meeting point gets this name.
Part B: Matching
Match each item on the left to the correct answer on the right.
1. Match each item to its correct answer.
Railroad tracks
→ Parallel lines
Line segment
Corner of a book
→ Perpendicular lines
Perpendicular lines
Flashlight beam
→ Ray
Ray
Edge of a ruler
→ Line segment
Parallel lines
Correct matches: Railroad tracks → Parallel lines; Corner of a book → Perpendicular lines; Flashlight beam → Ray; Edge of a ruler → Line segment.