What is a Triangle?
In geometry, a triangle is a closed, twodimensional shape with three straight sides. A triangle is also a polygon.
We can find the shape of a triangle in a flag, the musical instrument triangle, and a roadside signboard.
Properties of a Triangle
 A triangle has three sides, three vertices, and three angles.
 The sum of the three interior angles of a triangle is always 180°.
 The sum of the length of two sides of a triangle is always greater than the length of the third side.
 A triangle with vertices P, Q, and R is denoted as △PQR.
 The area of a triangle is equal to half of the product of its base and height.
Different Types of Triangles
To classify triangles according to their angles, we measure each of its interior angles. Triangles can be classified by angles, as:



An acute triangle has all interior angles acute (less than 90°), a right triangle has one right angle (equal to 90°) and an obtuse triangle has one obtuse angle (greater than 90°).
To classify the triangles according to their sides, we measure the length of each of its sides. Triangles can be classified by their sides, as:



To classify triangles according to both angles and sides, we measure the interior angles and length of the sides of the triangle. Few examples of triangles classified on the basis of both angles and sides are:



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