Building Geometric Measurements

Creating Geometric Angles plays a vital role in the study of geometry, as it serves as a foundation for constructing various other geometric shapes, particularly triangles. Before delving into the construction of angles using a protractor, let us take a moment to review angles and their classifications. An angle forms when two rays originate from a common point, known as the vertex, while the two rays themselves are referred to as arms or sides. Depending on the degree of inclination between its arms, an angle can be classified as acute (less than 90 degrees), obtuse (more than 90 degrees), or right-angled (exactly 90 degrees).

In this piece, we shall explore:

• The construction of angles with given measurements and unknown measurements, utilizing geometric tools such as the
  • Protractor
  • Compasses
  • Ruler
• The process of constructing a 75-degree angle

Additional resource: For more examples of constructing various angles, visit Examples of Construction of Different Angles

The Function of the Protractor

The protractor is a semicircular tool used to draw and measure angles. It is graduated from 0 to 180 degrees, allowing for the direct measurement of angles within this range. The protractor comprises two sets of markings, one ranging from 0 to 180 degrees from left to right, and the other vice versa.

Art of Constructing Angles Using a Protractor

When it comes to constructing angles of any given measurement, whether they are acute, obtuse, or right-angled, the easiest method involves the use of a protractor. To illustrate, let's assume you are tasked with constructing a 120-degree angle. The following steps are involved:

Step 1: Begin by drawing a line segment BC, which will serve as one of the arms of the angle to be constructed.

Step 2: Position the protractor's point O on point B of the line segment BC.

Step 3: Align OQ with the BC edge.

Step 4: The protractor consists of two sets of markings. We will consider the scale that begins with 0 degrees near point C for construction. Mark point A next to the 120-degree marking on the scale.

Step 5: Connect points A and B to form the angle ∠ABC, which measures 120 degrees.

Constructing Angles Using Compasses

The process of constructing angles with unknown measurements is essentially the act of duplicating a given angle where the measurement is not provided. Compasses are the primary tool utilized for this task. For instance, if you are given ∠BAC and need to replicate it, follow these steps:

Here are the instructions for constructing angles using a compass:

Procedure 1:

Begin by drawing a line PQ. Point P will serve as the vertex of the angle you want to replicate.

Procedure 2:

Position the compass pointer at point A and create an arc that intersects arms AC and AB at points K and J, respectively.

Procedure 3:

Without changing the compass radius, draw an arc on line PQ at point M.

Procedure 4:

Adjust the compass so that the pointer is placed at K and the pencil head is positioned at J.

Procedure 5:

Using the same radius, draw an arc on the initial arc with the compass pointer at M. Mark the point where they intersect as L.

Procedure 6:

Connect points P and L with a ruler, extending the line until you reach point R.

Procedure 7:

The angle ∠RPQ is the desired angle.

Instructional Video on Angle Construction

For more in-depth guidance on constructing angles, please refer to the following video:

How to Construct an Angle of 75 Degrees

To construct a 75-degree angle, follow these steps:

• Procedure 1: Begin by drawing a line segment with endpoints O and A.
• Procedure 2: Using a compass, draw an arc with O as the center that intersects the line segment OA at point B.
• Procedure 3: Maintaining the same radius, draw an arc with B as the center, cutting the previous arc at point C.
• Procedure 4: Keeping the radius unchanged and using C as the center, draw an arc that intersects the previous arc at point D.
• Procedure 5: Using any radius, draw two arcs with C and D as centers. The intersection of these arcs will be point E.
• Procedure 6: Connect points O and E with a ruler. This line, OE, will form a 90-degree angle.
• Procedure 7: Now, extend the line OE to intersect the arc at point F.
• Procedure 8: Taking F and C as centers, draw an arc with a radius greater than half the measurement of FC. The arc will intersect at point H.
• Step 9: Connect point H with point O. The angle HOA is determined to be 75 degrees.
• Step 10: The angle HOA is the angle we want to achieve.


For further insights into the art of angle construction, you can access BYJU'S - The Learning App.