A 180-degree angle is a straight angle that forms a straight line and measures exactly 180 degrees. It divides a full 360-degree rotation into two equal halves and is a fundamental concept in geometry.

Definition

In geometry, a 180-degree angle is called a straight angle because its arms extend in opposite directions, forming a straight path. It is one of the standard angle classifications, along with acute, obtuse, and right angles.

Types of Angles in Geometry

Here is a quick overview of angle types to understand how the 180-degree angle fits:

Characteristics of a 180-Degree Angle

1. Appearance: Resembles a straight line.
2. Measurement: Always equals 180 degrees.
3. Complementary Nature: Divides 360° into two halves.
4. Relation to Other Angles:
   • Two right angles (90° each) form a straight angle.
   • A straight angle is supplementary to itself.

Examples of a 180-Degree Angle in Real Life

• The edge of a perfectly straight ruler or table.
• A flat highway stretching infinitely in both directions.
• The time shown as 6:00 on an analog clock.

How to Measure a 180-Degree Angle with a Protractor

1. Draw a straight line (the first arm of the angle).
2. Place the protractor's center on the vertex of the angle.
3. Align the baseline of the protractor with the drawn line.
4. Mark the point at 180° on the protractor.
5. Join the vertex to the marked point to form the second arm.

How to Construct a 180-Degree Angle Using a Compass

1. Draw a straight line and label it as XY.
2. Choose a point O on the line as the vertex.
3. Place the compass at O and draw an arc across the line, marking points P and Q.
4. The resulting angle POQ is the required 180-degree angle.

Solved Examples

Example 1:
Problem: If a straight angle is divided into two parts, and one part measures 70°, what is the other part?
Solution:
A straight angle = 180°
Let the unknown angle = x
$x+70=180$
$x=180-70$
$x=110°$
Answer: The other angle is 110°.

Example 2:
Problem: Two angles are supplementary, and one measures 130°. Find the other angle.
Solution:
Supplementary angles = 180°
Let the unknown angle = x
$x+130=180$
$x=180-130$
$x=50°$
Answer: The other angle is 50°.

FAQs

Q1. What is a 180-degree angle called?
A: A 180-degree angle is called a straight angle.

Q2. How does a 180-degree angle look?
A: It looks like a straight line.

Q3. Can a triangle have a 180-degree angle?
A: No, but the sum of a triangle's interior angles equals 180 degrees.

Q4. What is the difference between a straight angle and a reflex angle?
A: A straight angle is exactly 180°, while a reflex angle is greater than 180° but less than 360°.