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E:\Working\Focus_Learning\Math_Genius_5_(05-10-2023)\Open_Files\CHAP_06
\\November 23, 2023 10:09 AM Surender Prajapati Proof 5 Reader _________________________ Date: ___________________74
Remember
Polygons are closed plane figures made up of only line segments which do not cross each other.
Each side of the polygon intersects exactly two other sides at the end points only.
No two sides cross each other.
trianGle
A
A triangle is a closed plane figure bounded by three line segments.
It is a three sided polygon.
The symbol ‘D’ is used to represent a triangle. The triangle ABC
shown alongside is written as DABC.
A triangle has three sides, three angles and three vertices. B C
The following table tells about the parts (sides, angles, vertices) of DABC.
Part Number of Parts Name
Sides 3 AB, BC, CA
Vertices 3 A, B, C
Angles 3 ∠BAC or ∠A, ∠ABC or ∠B, ∠ACB or ∠C
types of triangles
We know that a triangle has three sides and three angles. Hence, triangles are classified
on the basis of their: (i) sides (ii) angles
on the Basis of sides
A
equilateral triangle: A triangle whose all three sides are of equal length
is called an equilateral triangle. cm 3
In the adjoining figure, DABC is an equilateral triangle, as AB = BC 3 cm
= CA = 3 cm.
Also, all the angles are equal in an equilateral triangle which are equal B 3 cm C
to 60°.
For example, in DABC, ∠A = ∠B = ∠C = 60°. A
Isosceles triangle: A triangle whose two sides are of equal length is
called an isosceles triangle. 3 cm 3 cm
In the adjoining figure, DABC is an isosceles triangle, as
AB = AC = 3 cm, and BC = 4 cm. B 4 cm C
In an isosceles triangle, the angles opposite to equal sides are also equal. In DABC, ∠B = ∠C.
Scalene triangle: A triangle whose all three sides are of different lengths A
is called a scalene triangle.
In the adjoining figure, DABC is a scalene triangle, as AB = 2 cm, 2 cm 3 cm
BC = 4 cm and CA = 3 cm. All the angles in a scalene triangle are of
different measures. B 4 cm C
138 Mathematics-5
