A triangle is a closed figure made by three line segments. It has three vertices, three sides and three angles.


A triangle ABC is given having three vertices A, B, C and three sides AB, BC, CA and three angles are  ABC,  BCA,   CAB.

Classification of triangles:
1.    With respect to angles are:
    (a)     Acute-angled triangles 
    (b)     Obtuse-angled triangles 
    (c)     Right-angled traingles 

(a)     Acute-angled triangles


A triangle in which measurement of each of the three angles are less than 90°.

(b)     Obtuse-angled triangle :


A triangle in which one angle is obtuse angle i.e. greater than 90° but less then 180°.

(c)     Right-angled triangle:


A triangle in which one angle is exactly 90°.
        •    The side opposite to the right angle is called the hypotenuse.
        •    The other two sides are called as the legs of the right-angled triangle (or base and perpendicular).
        •    In the given figure AC is the hypotenuse and AB and BC are the legs of  DABC.