How do you define a triangle!!

There are four things which remain constant in any triangle

1. It is a polygon with three sides. (Consists three internal and three external angles)
2. Sum of internal angles is equal to 180 degrees.
3. Sum of any two sides is greater than the third one.
4. Difference between any two sides is less than the third one.

No matter what type of triangle we have these four things always holds true.

Perimeter of a triangle is sum of all the sides of a triangle.

Types of triangle;

According to angles

1. Acute angle triangle – where all angles in a triangle are less than 90 degrees.
2. Right angle triangle – where one of the angles is equal to 90 degrees.
3. Obtuse angle triangle – where one of the angles is greater than 90 degrees.

Important properties:

In a right angle triangle above, right angled at Q, by using Pythagoras theorem we can say that c² = a² + b²

For an obtuse angle triangle having sides a units, b units and c units, where side with c units is the obtuse side, we can say that c² > a² + b²

And, for an acute angle triangle having sides a units, b units and c units, we can say that c² < a² + b²

According to sides

1. Equilateral triangle – All sides equal and all internal angles equal to 60°.
2. Isosceles triangle – Any two sides and the angles opposite to them are equal.
3. Scalene triangle – None of the sides or angles are equal.

General formulas for equilateral triangle;

General formulas for right angle triangle;

In case of right-angle triangle as shown above, we can consider base of triangle as PQ or as QR and other one will automatically be considered as height of triangle.

Area of right-angle triangle (1/2) x (height) x (base) = (1/2)x a x b

General formulas for isosceles triangle;

In the isosceles triangle above, the two equal sides are equal to “a” unit and the unequal sides is equal to “b” unit. Consider the height of the triangle to be “h”. We can calculate height and area of triangle using the formula as;

Try the below questions once you have gone through the above video;