(i) A triangle is a plane figure formed by three non-parallel line segments.
(ii) The three sides and the three angles of a triangle are together known as the parts or elements of that triangle.
(iii) A scalene triangle is a triangle in which no two sides are equal.
![](/img/study_content/content_ck_images/images/r_d_triangle_15_1_scalene.png)
(iv) An isosceles triangle is a triangle in which two sides are equal.
![](/img/study_content/content_ck_images/images/r_d_triangle_15_1_Isosceles.png)
(v) An equilateral triangle is a triangle in which all three sides are equal.
![](/img/study_content/content_ck_images/images/r_d_triangle_15_1_equilateral.png)
(vi) An acute triangle is a triangle in which all the angles are acute (less than 90
).
![](/img/study_content/content_ck_images/images/r_d_triangle_15_1_Acute.png)
(vii) A right angled triangle is a triangle in which one angle is right angled, i.e 90
.
![](/img/study_content/content_ck_images/images/r_d_triangle_15_1_Right.png)
(viii) An obtuse triangle is a triangle in which one angle is obtuse (more than 90
).
![](/img/study_content/content_ck_images/images/r_d_triangle_15_1_Obtuse.png)
(ix) The interior of a triangle is made up of all such points that are enclosed within the triangle.
(x) The exterior of a triangle is made up of all such points that are not enclosed within the triangle.