Mr. Lourivaldo da Silva

Mathematics

Sum of interior angles of the triangle

* Michele France Viana Debus *
 Special to Page 3 Communication & Education

Objectives

Using a practical activity, leading the student to:
1) conclude that the sum of interior angles of the triangle is 180 degree.
2) recognize the characteristics of the external angle of the triangle.

Strategies

Before showing the properties of the angles of the triangle and the property of the exterior angle is important to explain to the class the following contents: angles (classification: acute, obtuse, shallow, etc..) And triangles (its elements).

Activities

1) Ask students to draw and cut out three equal triangles of any kind in waxed paper or in a role more firmly, yet easy to handle.

2)They should mark the angles with three different brands. All triangles should be identical.

3) At this point, the teacher can discuss the congruence of triangles or the concept of congruence in geometry.

4) Instruct the students to put together  one vertex of each triangle, so that the three angles are adjacent, two by two.

5) Ask students what are they concluding:

What are the forms of the three angles?

What is the measure of this new angle?

 Where those three angles come from?

 What happen if we exchange the angles of each triangle by putting them all together with the angles of the other triangles?

6) After this discussion, ask them to draw a line on paper, with the help of the ruler, and fix one of the triangles with the base on this line. After the triangle set, discuss the concept of the external angle.
7) On the line, it is possible to identify two external angles. Instruct the students to put one of the identified external angles, and other angles of the two other triangles, on the outer corner angle of each of the two triangles that haven't been set yet.  And ask:

Which angles fit perfectly in the external angle?

What does it means?

What relationship can be found between the external angle of the triangle and the other angles?

 

Michele Viana Debus France has a degree in mathematics from USP and an MA in mathematics education at PUC-SP.