Angles and Angle Measurement — Answer Key
Part A: Fix the Sentence
Each sentence has an error. Rewrite it correctly on the line.
1. Fix the sentence:
The angle measures 120° because the ray points to 120 on the protractor.
Corrected: The angle measures 60° because you must read from the scale that starts at 0° on the base ray.
The corrected sentence is: "The angle measures 60° because you must read from the scale that starts at 0° on the base ray.". The original sentence "The angle measures 120° because the ray points to 120 on the protractor." contained an error that needed to be fixed.
2. Fix the sentence:
A straight angle measures 360° because the rays form a line.
Corrected: A straight angle measures 180° because the rays point in opposite directions forming a line.
The corrected sentence is: "A straight angle measures 180° because the rays point in opposite directions forming a line.". The original sentence "A straight angle measures 360° because the rays form a line." contained an error that needed to be fixed.
3. Fix the sentence:
An angle of 75° is a right angle because it is close to 90°.
Corrected: An angle of 75° is an acute angle because it is less than 90°.
The corrected sentence is: "An angle of 75° is an acute angle because it is less than 90°.". The original sentence "An angle of 75° is a right angle because it is close to 90°." contained an error that needed to be fixed.
Part B: Fill in the Blank
Write the missing word or number on each line.
1. A protractor measures angles in units called degrees.
Degrees (°) are the standard unit for measuring angles — a full rotation is 360 degrees.
2. To measure an angle, place the center of the protractor on the vertex.
The vertex is the point where the two rays of an angle meet — the protractor center aligns here.
3. An angle measuring exactly 90° is called a right angle.
A right angle measures exactly 90° and is marked with a small square symbol.
4. A full turn around a point measures 360 degrees.
One complete rotation equals 360°.
Part C: Short Answer
Answer each question in one or two complete sentences.
1. Explain the two steps for placing a protractor to measure an angle.
Sample answer: First, place the center point of the protractor on the vertex of the angle. Then align the base line of the protractor with one ray of the angle.
Proper protractor placement (center on vertex, baseline on one ray) is essential for accurate angle measurement.
2. Why does a protractor have two scales of numbers?
Sample answer: A protractor has two scales so you can measure an angle from either direction. You use the scale that starts at 0° on the ray you aligned.
The inner and outer scales allow measurement of angles opening in either direction.