Straight Angle

A straight angle is a part of geometry. You must have learned about the basics of geometry which consist of angles, ray, lines, line segments, etc. Here you will learn complete demonstration of one of the type of angle, which is a straight angle.

Different Types of Angles

An angle is formed when two line segments are joined at a single endpoint or when two line segments are combined at a single endpoint. The point where the two lines meet is called the Vertex of the angle. There are basically 6 different types of angles whose vertex values are different, respectively and they are;

  • Acute Angle(less than 90°)
  • Obtuse Angle(more than 90°)
  • Right Angle(equal to 90°)
  • Straight Angle(exact 180°)
  • Reflex Angle(greater than 180°) and
  • Full Angle(Exact 360°)

An angle with 0° is called a zero angle, means it doesn’t form an angle. These are the general classification of angles.

In geometry, we compare two angles based upon their conditions. Some angles are defined in terms of their similarity, called as a congruent angle, which has the same angle. Then, we have adjacent angles, which are adjacent to each other. Complementary angles, whose sum is equal to 90°.

In this article, we are going to brief you about “Straight Angle” theorem with its definition, its mathematical value and example.

Straight Angle Definition

In geometry, a straight angle is an angle, whose vertex point has a value of 180 degrees. Basically, it forms a straight line, whose sides lie in opposite directions from the vertex. It is also termed as “flat angles”


Straight Angle

Straight Angle Theorem

Straight Angle theorem states that all the straight angles are 180 degrees. If the legs are pointing exactly in the opposite directions, then it forms a straight angle. A straight angle is represented as by 180° or π.

Properties of Straight Angles

Some important properties of straight angles are as follows:

  • A straight angle measures exactly half of a revolution
  • A straight angle is produced by revolving one ray by 180°with respect to another ray
  • In a straight angle, the angle rays face in the opposite orientations
  • A straight angle modifies the direction of a point
  • A straight angle can also be formed by joining two right angles

Straight Angle Examples

Some of the straight angle examples in our day to day life are:

  • A flat surface has an angle of 180 degrees
  • A straight stick has an angle which is straight or 180 degree
  • A plane inclined staircase represents a straight angle
  • Angle formed in a see-saw

A straight angle is different from the straight line, as it measures 180 degrees and a straight line is a connector of two points.

Can we Consider a Triangle Made from a Straight Angle?

A straight angle has a measure of 180°and the sum of interior angles of a triangle is also 180°. But we cannot consider straight angle as a straight angle triangle because, for a triangle, the figure should form an enclosed structure.

Triangle is a closed three-sided figure, which has three distinct points which are not collinear with each other.  But, in the case of the Straight line, the points are collinear and hence they cannot form a closed structure. Therefore, there is no triangle formed.

How to construct a Straight Angle?

The steps to construct a Straight Angle are as follows:

  • Draw a straight-line OA using a ruler, with an arrowhead at A
  • Now place the protractor on this line such that the baseline of the protractor is adjusted over OA. The point A should be facing towards the 0°of the protractor.
  • Now mark the point, Starting from 0° and moving towards 180 degrees. On reaching 180°, mark a point B on the paper.
  • Now, join the vertex of the line OA with the marked point B. The second ray OB should have arrowhead at B.

How to Construct a Straight Angle?

Points to remember:

  • A straight angle always forms a straight line.
  • The value of the straight angle is 180°.
  • The arms of the straight angle lie opposite to each other from the vertex point.
  • The straight angle measured anticlockwise is positive straight angle i.e.180°.
  • The straight angle measured clockwise is a negative straight angle i.e.-180°.

Frequently Asked Questions

What is the Angle of a Straight Angle?

The angle of a straight angle is 180°.

What is Half of a Straight Angle?

The half of a straight angle is a right angle. Since, a straight angle is 180 degrees, the half of it will be 90 degrees which is a right angle.

Is a straight line a reflex angle?

No, a straight line is a straight angle and not a reflex angle. It should be noted that a reflex angle measures more than 180° and less than 360°.

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