Kinds of Angles
Angles are classified by their measure in degrees: acute angles are less than 90 degrees, right angles are exactly 90 degrees with perpendicular sides, obtuse angles are between 90 and 180 degrees, and straight angles are exactly 180 degrees forming a straight line. These classifications are foundational vocabulary for all geometry work. This Grade 8 math skill from Yoshiwara Core Math Chapter 1 introduces students to angle terminology and gives them tools to describe and analyze geometric figures. Recognizing angle types is prerequisite knowledge for work with triangles, polygons, parallel lines, and trigonometry.
Key Concepts
Property When two line segments meet at a point, they form an angle. The point where the two sides meet is called the vertex.
If the sides are less open than a right angle, we call the angle acute. If the sides are more open than a right angle, we call the angle obtuse. If the two sides are totally open to form a straight line, we call the angle a straight angle. In a right angle, the sides are perpendicular.
Examples The tip of a slice of pie typically forms an acute angle. At 5:00, the hands of a clock form an obtuse angle. A perfectly straight pencil lying on a desk represents a straight angle.
Common Questions
What are the types of angles?
The four main types of angles are: acute (less than 90 degrees), right (exactly 90 degrees), obtuse (between 90 and 180 degrees), and straight (exactly 180 degrees). A reflex angle is greater than 180 degrees.
What is an acute angle?
An acute angle measures less than 90 degrees. The word acute means sharp, describing the narrow opening. Examples include the tip of a pizza slice, a corner of a triangle less than 90 degrees, and 3 o'clock on a clock.
What is a right angle?
A right angle measures exactly 90 degrees. The sides of a right angle are perpendicular to each other. Right angles are marked with a small square in diagrams. Corners of squares, rectangles, and most rooms are right angles.
When do 8th graders learn about kinds of angles?
Students study types of angles in Grade 8 math as part of Chapter 1 of Yoshiwara Core Math, which covers preliminary ideas in geometry including angle classification.
What is an obtuse angle?
An obtuse angle measures between 90 and 180 degrees. It is wider than a right angle. Examples include the angle of a reclining chair back, the hour hand at 8 o'clock, and an interior angle of a regular hexagon (120 degrees).
What is a straight angle?
A straight angle measures exactly 180 degrees. Its two rays point in exactly opposite directions, forming a straight line. A straight angle looks like a flat line with a vertex in the middle.