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Properties and Solutions of Triangles for IIT JEE

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Last updated date: 29th May 2024
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Overview of Triangle



A triangle is a polygon that is a three sided figure. A triangle has basically three sides and three vertices. When three co linear points are there in the plane, then joining them you can form a triangle.


Every triangle has three vertices, along with this they have three edges. You can find the area of a triangle by ½ × base × height formula. That is, the length of base and height of the triangle are multiplied and the result is divided into two. You can easily find the perimeter of the triangle by summing the side length of the triangle. Internal angle of the triangle is measured to be 60 degrees. Whereas the sum of all the interior angles of the triangle is 180 degrees.


If we talk about triangles in general then they are three types that are acute angle triangle, right angle triangle and obtuse angle triangle. In an acute angled triangle, all the angles of the triangle are less than 90 degrees. In a rightIn a right angle triangle one angle of the triangle is equal to 90 degrees. An obtuse angle triangle measures if one angle of a triangle is definitely greater than 90 degrees.


Now let us learn the concept of triangles in detail. Here in this article you are provided with the details of the triangle topic such as how many types of triangle there are, properties of the triangle and along with this the solutions of the problems related to triangles. In the end you will come across Frequently Asked Questions that will help you to clear most of your doubts about this topic and clarify your concept on a wider basis.


Types, Properties and Solutions of Triangles

In thisIn this chapter we are going to discuss Triangle properties and also its solutions. We all know that a triangle consists of three angles & three sides. It is a closed figure made up of three line segments. 


In the figure above, the three line segments are AB, BC, CA and the three angles are denoted as ∠​A, ∠​B, ∠​C. 


The triangle can be classified into three different types based on the sides. These are as follows:

  1. Isosceles triangle 

  2. Scalene triangle

  3. Acute-angled triangle 

  4. Obtuse-angled triangle  

  5. Right-angled triangle  

1. Isosceles Triangle: A triangle that consists of 2 sides of equal length is known as an Isosceles triangle. In this, the two angles opposite to the equal sides are equal. 

2. Scalene Triangle:​ A Scalene triangle is a kind of a triangle that consists of three sides of different lengths.  

3. ​Acute-Angled Triangle:  A triangle in which all the angles are acute is called an acute-angled triangle. It is also called the Acute triangle. 

4. Obtuse-Angled Triangle:  A triangle in which one of the angles is obtuse is called an obtuse-angled triangle. In this triangle one angle is obtuse and the other two are acute angles. TheThe measure of the angles is in such a way that it is equal to the sum of 180 degrees.

5. Right-Angled Triangle:  A triangle in which one of the angles is a right-angle is called a right-angled triangle or right triangle. In the figure below, the side opposite to the right angle, BC is called the hypotenuse. In a right angled triangle it is necessary that one angle is of 90 degrees measurement while the other angles can measure less than 90 degrees in order to make the sun's interior angles if triangle to be 180 degrees. A Pythagoras theorem will help you to find out whether a triangle is right angled or not. Have a look at it. For the Right angled triangle ABC, BC​2= AB​2+ AC​2

This theorem is called as ​Pythagorean Theorem. 

In the above drawn triangle, 5​2= 3​2​+ 4​2. Those triangles which satisfy this specific condition are known as right angle triangles.


So, the Pythagorean Theorem helps to find whether a triangle is Right-angled (or) not 


There are different types of right angles. As of now, our focus is only on a special pair of right angles they are,

  1. Triangle 45-45-90  

  2. Triangle 30-60-90 

1. Triangle 45-45-90: As the name indicates, a 45-45-90 triangle, is the right triangle in which the other 2 angles are 45° each.

It is an isosceles right angle triangle.


(Image will be uploaded soon)


In this figure we can view that the angle ∆ DEF, DE = DF and finally ∠​D = 90°.


The sides values are 45-90-45 and the triangle is in the ratio 1: √2 : 1 


2. Triangle 30-60-90: A 30-60-90 triangle, as the name indicates, is the right triangle in which the other two angles are 30° and 60°. It is a scalene right triangle in which none of the sides or the angles are equal.


The sides in a 30-60-90 triangle are in the ratio 1 : √3 : 2


Like other right triangles, these two angles satisfy the Pythagorean Theorem.  Now, let us discuss about the properties of triangle in details:

  • The first property is angle-sum property. In this, the sum of the angles in a triangle is 180°.

  • The total sum of the lengths (L) of any two sides of a triangle is greater than the length of the third side. Like the difference between the lengths of any two sides of a triangle is less than the length of the third side. 

  • The side opposite to the largest angle is the longest side of the triangle & the side opposite to the smallest angle is the shortest side of the triangle. 


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  • In the figure above, the angle ∠​B is the largest angle and the side which is opposite to it, that is, hypotenuse, is the largest side of the triangle.

  • In the figure above, the angle ∠​A is the largest angle and the side opposite to it, the portion BC is the largest side of the triangle.

  • Exterior angle property of a triangle: It is an exterior angle of a triangle which is equal to the sum of its interior opposite angles. 


(Image will be uploaded soon)


  • Here, the angle ∠​ACD is the exterior angle to the ∆ABC.

  • Exterior angle property is as ∠​ACD = ∠​CAB + ∠​ABC.


One More Important Point is:

1. The shortest side of a triangle is always found opposite to the smallest interior angle of a triangle. 1. And, alternatively one more consequent point is that the longest side is always found to be opposite to the largest interior angle of a triangle. Now, we discuss about the Solutions of Triangle: In typical point of view, a triangle has six main characteristics. It contains three linear (side lengths ​a​, ​b​, ​c​) and three angular (​α​,​ β​,​ γ​). The classical plane trigonometry problem is to specify three of the six characteristics and to determine the other three. 

The triangle can be specifically determined in this method when given any of the following:

  • Three sides (​SSS​) 

  • A side, the angle adjacent to it and an angle also opposite to it (​AAS​). 

  • 2 sides of a triangle are included and an angle not included between them (​SSA​), if the side length adjacent to the angle is lesser than the other side length.  

  • A side and the 2 angles are found adjacent to it (​ASA​) 

  • 2 sides and the included of angles of triangle (​SAS​) 

  • Finally, 3 angles (​AAA​) on the sphere (but not in the plane).

In the plane for all the cases, at least one of its side lengths must be specified. At the time if only the angles are provided, the side lengths will be unable to be determined, because any similar ​kind of a triangle is the solution.

In this we are going to discuss about the important 3 characteristics as follows: 

(1) ​Side-Side-Side (SSS) criterion for congruence​:  If all the three sides of a triangle are equal to the respect to three sides of another triangle, then that triangle angles are said to be congruent.

Now, ∆ ABC ≅​∆ XYZ as AB = XY, BC = YZ and AC = XZ. 


(2) ​Side-Angle-Side (SAS) criterion for congruence​: If only 2 sides and the angle included between the 2 sides of a triangle are equal to the respective 2 sides and the included angle of another triangle, then the triangles are congruent.

Now, ∆ ABC ≅​∆ XYZ as AB = XY, ∠​A = ∠​X and AC = XZ. 


(3) ​Angle-Side-Angle (ASA) criterion for congruence: If 2 angles and the included side of a triangle are found to be same to the corresponding 2 angles and the considered side of other triangle, this kind of triangle is said to be congruent.

We, found in above figure that, ∆ ABD ≅​∆ CBD in which (approx.) the values of ∠​ABD =∠​CBD, the values of AB = CB and the values of∠​ADB =∠​CDB 

Let's enter into discussion of triangle area calculation with the following simple formula: 


Triangle Area Calculation

Triangle is also known as a polygon with the three edges and the three vertices. A triangle is one of the basic shapes of geometry. 


Area of a triangle can be calculated by simple formula mentioned below:The Area of a triangle = (1/2) *Value of Base *Value of Height


To find the area of a triangle, we draw a straight line (or) perpendicular line from the base to the opposite vertex which provides the height of the triangle. 


Hence, the area of the ∆ PQR = (1/2) * (PR * QS) = (1/2) * 6 *4 =12 sq. units. 


For a right angled triangle, it is easy to determine the area as there is a side perpendicular to the base of the triangle, so we can consider it as a height.


The H, which means Height of the ∆ XYZ is XY and its area is (1/2) * XZ * XY and is measured in Sq.Units. Now, we can discuss finding the area of an obtuse triangle LMN?  


In case of an obtuse triangle, when we extend the base and draw a line perpendicular from the vertex to the extended base, it becomes the height of the triangle.


Now, the area of the triangle ∆ LMN = (1/2) * LM(base) * NK(height) sq. units.

It will be clearly known with the help of the following simple example:

  1. If ∆ ABC = 3 (∆ DEF), see the below statements and find the correct one?

  2. A. ∠​E = ∠​F = 120°, ∠​D = 40° AND DE = DF = 3 and EF = 2

  3. B. ∠​E = ∠​F = 110°, ∠​D = 40° AND DE = DF = 3 and EF = 2

  4. C. ∠​F = ∠​E = 40°, ∠​D = 100° AND DF = DE = 2 and EF = 3

  5. D. ∠​E = ∠​F = 110°, ∠​D = 40° AND DE = DF = 3 and EF = 3

Solution: C  


Definition for Perimeter 

A triangle perimeter can be defined as a total distance around outside of the triangle which can be determined by adding together the length of each side as per the formula: 

Perimeter = a+b+c

FAQs on Properties and Solutions of Triangles for IIT JEE

1. How can you identify a triangle?

When we classify any triangle by its sides you can look for the triangle having two sides same in length, or  having two sides different in length. If the length of the triangle is not the same then it will be known as scalene triangle. In the same way if two sides are similar in length it is an isosceles triangle and if all the sides are the same in length then it is an equilateral triangle.

2. Mention any four properties of a triangle.

Some of the properties of the triangles are mentioned below, have a look at them

  • A triangle has three sides and three vertices in a plane.

  • The sum of the angles of the triangle is always equal to 170 degrees.

  • The exterior angle of the triangle always sums up to be equal to 360 degrees.

  • The sum of consecutive angles, that is consecutive exterior and interior angles is equal to 180 degrees, that is, they are supplementary angles.

3. Why are triangles called triangles?

A polygon is a closed figure which has three or more sides in common. On the basis of the number of sides a polygon is having, they are given specific names. Let us take an example, the polygon with the three sides will be referred to as a triangle. This is because the prefix tri means three. This name indicates that the polygon figure has three sides and along with it three angles and three vertices also.

4. How can we find the hypotenuse of the triangle?

Hypotenuse is the longest side of the right angled triangle. Right angle triangle is that triangle which has one angle 90 degrees and other angles are acute angles. The sum of these angles will sum up to be 180 degrees. The sum of all the triangles is equal to 180 degrees. To find the hypotenuse side of the right angled triangle, you can use the formula, that is, square of hypotenuse is equal to the square of base + square of the altitude. From this hypotenuse formula can be derived which is equal to the square root of the square of base + square of 5he height of the triangle.

5. What are the benefits of using the notes of triangle properties from the Vedantu app?

Vedantu app provides you with the details of all the concepts. All the concepts are well discussed by the experts. Experts have specially designed these notes for you to help you to understand the concepts well, so that you can get knowledge of the concept and beside this they provide you with the tactics so that you can score good in your examination.Vedantu app is the most popular and prescribed  mostly by your teachers and guides for  your help to get a brief knowledge about the topic. At vedantu our main motive is to increase the capability of the students and improve their performance.