What is a Triangle ?

• It is a 3-sided polygon
• It has 3 sides , 3 vertices and 3 angles.
• It has no diagonals.

What are the Properties of a Triangle?

• Sum of all interior angles of a triangle = 180°
• Equilateral triangle has all 3 sided of equal length and each angle of measure 60°
• In a right triangle, with the side opposite the right angle being termed the hypotenuse and the other two sides being known as the legs.
• All triangles are convex and bicentric. 
• That portion of the plane enclosed by the triangle is called the triangle interior, while the remainder is the exterior.

• Perimeter of triangle

• Area of triangle 

Types of Triangles

1) Types of Triangles based on measure of Angles

Acute Triangle: All angles are less than 90°

Right Triangle: Has a right angle (90°)
Obtuse Triangle: Has an angle more than 90°

2) Types of Triangles based on measure of Sides

Equilateral triangles are triangles in which all three sides are the same length.
Isosceles triangles are triangles in which two of the sides are the same length.
Scalene triangles are triangles in which none of the sides are the same length.

Note :

• An angle cannot be obtuse and equilateral at the same time.
• An equilateral triangle cannot have an obtuse angle because all 3 angles in an equilateral triangle measure 60 degrees.