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Construct An Angle Bisector

In these lessons, we will learn
  • how to construct an angle bisector of a given angle.
  • how to use an angle bisector to construct some angles for example, 90 degrees, 45 degrees, 60 degrees, 30 degrees, 120 degrees, 135 degrees, 15 degrees.

Related Topics: More Geometry Lessons

An angle bisector is a straight line that divides the angle into two equal parts.

Example : Construct an angle bisector for the following angle:



Step 1 : Put the sharp end of your compasses at point B and make one arc on the line BC (point S) and another arc on line AB (point T).

Step 2 : Without changing the width of your compasses, put the sharp end of the compasses at S and make an arc within the lines AB and BC. Do the same at T and make sure that the second arc intersects the first arc.

Step 3 : Draw a line from point B to the points of intersection of the 2 arcs. This line bisects .

The following video shows how to draw an angle bisector using a compass and a straight edge.
1. Place compass point on the vertex, and draw an arc across each ray.
2. Place the compass on each arc intersection and draw a further pair of arcs which intersect each other.
3. Use a straight edge to connect the intersection point to the vertex.
The following video shows how to bisect a given angle using only a compass and straightedge.


Use angle bisector to construct angles

We can use the angle bisector to construct some other angles from existing angles.

Example :

A 30˚ angle can be obtained by bisecting a 60˚ angle.
A 15˚ angle can be obtained by bisecting a 30˚ angle.
A 45˚ angle can be obtained by bisecting a 90˚ angle.
A 22.5˚ angle can be obtained by bisecting a 45˚ angle.


The figure shows a point A on a straight line. Construct an angle of 45˚ at point A.


Construct a 90˚ angle, and then construct an angle bisector to obtain a 45˚ angle.

The following video shows how to construct 30, 45, 60, 90, and 120 degree angles with a compass

Rotate to landscape screen format on a mobile phone or small tablet to use the Mathway widget, a free math problem solver that answers your questions with step-by-step explanations.

You can use the free Mathway calculator and problem solver below to practice Algebra or other math topics. Try the given examples, or type in your own problem and check your answer with the step-by-step explanations.

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