Angles and Protractors
Grade 4 · Math · Study Guide
Learn how to identify, classify, and measure angles using a protractor.
An angle is formed when two rays share the same endpoint, called a vertex. Angles are measured in degrees (°). A full turn around a point is 360°.
Angles are classified by their size. An acute angle measures less than 90°. A right angle measures exactly 90° and is often marked with a small square. An obtuse angle measures more than 90° but less than 180°. A straight angle measures exactly 180° and looks like a straight line.
A protractor is a tool used to measure angles. To use one, place the center hole of the protractor on the vertex of the angle and line up one ray with the 0° line. Read the number where the second ray crosses the scale. Most protractors have two scales (inner and outer) going in opposite directions, so always start from the 0° mark on the same side as your angle opens toward.
Angles can be added together. If you know two angles share a common ray and together form a larger angle, you can add them. Two angles that together make 90° are called complementary angles. Two angles that together make 180° are called supplementary angles.
Key terms
- Angle
- A figure formed by two rays that share the same endpoint, measured in degrees.
- Vertex
- The point where two rays meet to form an angle.
- Acute angle
- An angle that measures less than 90°.
- Obtuse angle
- An angle that measures more than 90° but less than 180°.
- Right angle
- An angle that measures exactly 90°, often marked with a small square.
- Protractor
- A semicircular tool used to measure and draw angles in degrees.
Flashcards
- Acute angle
- Measures less than 90°; looks like a sharp, narrow opening
- Right angle
- Measures exactly 90°; marked with a small square; found in corners of squares and rectangles
- Obtuse angle
- Measures more than 90° but less than 180°; looks wider than a right angle
- Straight angle
- Measures exactly 180°; looks like a straight line with the vertex in the middle
- Reflex angle
- Measures more than 180° but less than 360°; the angle that 'wraps around' the other way
- How to use a protractor
- Place center on vertex, align one ray with 0°, read the scale where the second ray crosses
- Complementary angles
- Two angles whose measures add up to 90°; e.g., 30° and 60°
- Supplementary angles
- Two angles whose measures add up to 180°; e.g., 110° and 70°
Questions
An angle that measures 135° is called a(n) _____ angle.
- acute
- right
- obtuse
- straight
Show answer
Answer: obtuse
135° is greater than 90° but less than 180°, so it is an obtuse angle.
When using a protractor, you should always start reading from the 0° mark.
Show answer
Answer: True
You must align one ray of the angle with the 0° line on the protractor to get an accurate measurement.
Two angles are complementary if they add up to _____ degrees.
Show answer
Answer: 90
Complementary angles sum to 90°. For example, 30° and 60° are complementary because 30 + 60 = 90.
An angle measures 45°. What type of angle is it?
- Obtuse
- Straight
- Right
- Acute
Show answer
Answer: Acute
45° is less than 90°, so it is an acute angle.
A straight angle measures 180°.
Show answer
Answer: True
A straight angle forms a straight line; both rays point in opposite directions and the angle between them is 180°.
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