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:
A 120° angle is an acute angle because it is more than 90°.
Corrected: A 120° angle is an obtuse angle because it is more than 90° but less than 180°.
The corrected sentence is: "A 120° angle is an obtuse angle because it is more than 90° but less than 180°.". The original sentence "A 120° angle is an acute angle because it is more than 90°." contained an error that needed to be fixed.
2. Fix the sentence:
A right angle measures exactly 100°.
Corrected: A right angle measures exactly 90°.
The corrected sentence is: "A right angle measures exactly 90°.". The original sentence "A right angle measures exactly 100°." contained an error that needed to be fixed.
3. Fix the sentence:
A 45° angle is obtuse because 45 is less than 90.
Corrected: A 45° angle is acute because it measures less than 90°.
The corrected sentence is: "A 45° angle is acute because it measures less than 90°.". The original sentence "A 45° angle is obtuse because 45 is less than 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. An angle that measures less than 90° is called acute.
Acute angles measure between 0° and 89° — they are narrow angles.
2. An angle that measures exactly 180° is called a straight angle.
A straight angle measures exactly 180° and forms a straight line.
3. The corner of a book forms a right angle.
A right angle measures exactly 90° — it forms a perfect square corner like the corner of a book.
4. An angle measuring 135° is classified as obtuse.
Obtuse angles measure between 91° and 179° — 135° is between 90° and 180°.
Part C: Short Answer
Answer each question in one or two complete sentences.
1. How can you tell the difference between an acute angle and an obtuse angle?
Sample answer: An acute angle measures less than 90° and looks narrow, while an obtuse angle measures more than 90° but less than 180° and looks wide.
Comparing an angle to a 90° right angle is the quickest way to classify it as acute, right, or obtuse.
2. Name two objects in a classroom that have right angles.
Sample answer: The corner of a desk and the corner of a whiteboard both have right angles because they measure exactly 90°.
Right angles appear throughout everyday environments — recognizing them builds spatial reasoning.