CLASSIFYING TRIANGLES BY SIDES

About "Classifying triangles by sides"

Classifying triangles by sides :

In geometry, triangles can be classified using various properties related to their angles and sides.

In this section, we are going to study how triangles can be classified by their sides.  

Classifying triangles by sides

Each of the three points joining the sides of a triangle is a vertex. (The plural form of vertex is vertices) For example, in the triangle ABC shown below, points A, B and C are vertices.  

In a triangle, two sides sharing a common vertex are adjacent sides. In triangle ABC shown below, CA and BA are adjacent sides. The third side BC is the side opposite to ∠A.

Right and Isosceles Triangles

The sides of right triangles and isosceles triangles have special names. In a right triangle, the sides that form the right angle are the legs of the right triangle. The side opposite to the right angle is the hypotenuse of the triangle. 

An isosceles triangle can have three congruent sides, in that case it is equilateral. When an isosceles triangle has only two congruent sides, then these two sides are the legs of the isosceles triangle. The third side is the base of the isosceles triangle. 

Classifying triangles by sides - Examples

Example 1 :

Identify the type of triangle whose diagram is given below. 

Solution :

In the triangle given above, two of the sides are congruent. So, it is isosceles triangle. 

Example 2 :

Identify the type of triangle whose diagram is given below. 

Solution :

In the triangle given above, one of the angles right angle. So, it is right triangle.

Note : 

If one of the angles is right angle and two of the sides are congruent, it is right isosceles triangle. 

Example 3 :

Identify the type of triangle whose diagram is given below. 

Solution :

In the triangle above, the lengths of all the three sides are same and all the three angles are congruent. 

So, the given triangle is equilateral triangle. 

Example 4 :

Is it possible to have a triangle whose sides are 5 cm, 6 cm and 4 cm ? If so, identify the type of triangle. 

Solution : 

Verify the property

"The sum of the lengths of any two sides of a triangle is always greater than the third side",

for the given three sides. 

5 cm + 6 cm > 4 cm

6 cm + 4 cm > 5 cm

5 cm + 4 cm > 6 cm

Since the given sides meet the condition said in the property, it is possible to have a triangle whose sides are 5 cm, 6 cm and 4 cm.

All the three sides are different in length. So it is scalene triangle. 

Example 5 :

Is it possible to have a triangle whose sides are 7 cm, 2 cm and 4 cm ? If so, identify the type of triangle. 

Solution : 

Verify the property

"The sum of the lengths of any two sides of a triangle is always greater than the third side",

for the given three sides. 

2 cm  + 4 cm  <  7 cm

Since the given sides do not meet the condition said in the property, it is not possible to have a triangle whose sides are 7 cm, 2 cm and 4 cm.

After having gone through the stuff given above, we hope that the students would have understood, "Classifying triangles by sides". 

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