Looking Back

(1) (i) A triangle is a simple figure made up of three line segments.

(ii) Based on the sides, triangles are of three types:

A triangle having all three sides of length is called an Equilateral Triangle.

A triangle having at least sides of equal length is called an Isosceles Triangle.

If all the three sides of a triangle are of length, the triangle is called a Scalene Triangle.

(iii) Based on the angles, triangles are of three types:

A triangle whose all angles are is called an acute-angled triangle.

A triangle whose angle is is called an obtuse-angled triangle.

A triangle whose angle is a angle is called a right-angled triangle.

(2) The elements of a triangle are its three angles and the three sides.

(3) Properties of the lengths of the sides of a triangle:

(i) The sum of the lengths of any two sides of a triangle is than the length of the third side.

(ii) The difference between the lengths of any two sides of a triangle is than the length of the third side.

(4) The line segment joining a vertex of a triangle to the mid-point of its side is called a median of the triangle. A triangle has medians.

(5) The line segment from a vertex of a triangle to its opposite side is called the altitude of the triangle.

(6) The total measure of the three angles of a triangle is °. This is called the angle sum property of a triangle.

(7) The measure of any exterior angle of a triangle is equal to the of its interior opposite angles. This is called the exterior angle property of the triangle.

(8) Representation of line, line segment and ray:

LM = Length of Line segment of LM ;

LM = Line segment LM

LM = Ray LM ;

LM = Line LM