NCERT 5th Class (CBSE) Mathematics: Polygons And Circles

TRIANGLES

Properties of Triangles

A. Find the sum of the angles in these triangles.

In all three triangles, the sum of the measures of the angles is 180°. This is a property of all triangles.

The sum of the measures of the three angles of a triangles is always 180°.

You can check this out for yourself.

1. Draw and cut out a triangle.
2. Colour the three angles of the triangle.
3. Tear off the three corners of the triangle.
4. Place them together like this.

All three pieces fit together to from a straight angle.

∠1 + ∠2 + ∠3 = 180°

Look at triangles ABC. The measures of ∠A and ∠B are given. What is the measure of ∠C ?

In ΔABC, ∠A + ∠B = 60° + 80° = 140°

The sum of three angles of a triangle is 180°

So, ∠C = 180° – 140° = 40°

Answer: ∠C = 40°