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Maths ppt of class 9 CBSE topic Triangles

We know that a closed figure formed by three intersecting lines is called a
triangle(‘Tri’ means ‘three’).A triangle has three sides, three angles and three
vertices. For e.g.-in Triangle ABC, denoted as ∆ABC AB,BC,CA are the three sides,
∠A,∠B,∠C are three angles and A,B,C are three vertices.
A
B C

OBJECTIVESIN THIS LESSON
1
• INEQUALITIES IN A TRIANGLE.
2
• STATE THE CRITERIA FOR THE CONGRUENCE OF
TWO TRIANGLES.
3
• SOME PROPERTIES OF A TRIANGLE.
4
•DEFINE THE CONGRUENCE OF TRIANGLE.

DEFINING THE CONGRUENCE OF TRIANGLE:-
Let us take ∆ABC and ∆XYZ such that corresponding angles
are equal and corresponding sides are equal :-
X
Y Z
CORRESPONDINGPARTS
∠A=∠X
∠B=∠Y
∠C=∠Z
AB=XY
BC=YZ
AC=XZ
A
B C

Now we see that sides of ∆ABC coincides with sides of ∆XYZ.
A
B C
X
Y Z
SO WE GET THAT
TWO TRIANGLES ARE CONGRUENT, IF ALL THE SIDES AND ALL THE ANGLES
OF ONE TRIANGLE ARE EQUAL TO THE CORRESPONDING SIDES AND
ANGLES OF THE OTHER TRIANGLE.
Here, ∆ABC ≅ ∆XYZ

This also means that:-
A corresponds to X
B corresponds to Y
C corresponds to Z
For any two congruent triangles the corresponding parts are equal and are
termed as:-
CPCT – Corresponding Parts of Congruent Triangles

CRITERIASFORCONGRUENCEOF TWOTRIANGLES
SAS(side-angle-side) congruence
• Two triangles are congruent if two sides and the included angle of one triangle are equal
to the two sides and the included angle of other triangle.
ASA(angle-side-angle) congruence
• Two triangles are congruent if two angles and the included side of one triangle are equal
to two angles and the included side of other triangle.
AAS(angle-angle-side) congruence
• Two triangles are congruent if any two pairs of angle and one pair of corresponding sides
are equal.
SSS(side-side-side) congruence
• If three sides of one triangle are equal to the three sides of another triangle, then the two
triangles are congruent.
RHS(right angle-hypotenuse-side) congruence
• If in two right-angled triangles the hypotenuse and one side of one triangle are equal to
the hypotenuse and one side of the other triangle, then the two triangles are congruent.

A
B C
P
Q R
S(1) AC = PQ
A(2) ∠C = ∠R
S(3) BC = QR
Now If,
Then ∆ABC ≅ ∆PQR (by SAS congruence)

A
B C
D
E F
Now If, A(1) ∠BAC = ∠EDF
S(2) AC = DF
A(3) ∠ACB = ∠DFE
Then ∆ABC ≅ ∆DEF (by ASA congruence)

A
B C P
Q
R
Now If, A(1) ∠BAC = ∠QPR
A(2) ∠CBA = ∠RQP
S(3) BC = QR
Then ∆ABC ≅ ∆PQR (by AAS congruence)

Now If, S(1) AB = PQ
S(2) BC = QR
S(3) CA = RP
A
B C
P
Q R
Then ∆ABC ≅ ∆PQR (by SSS congruence)

Now If, R(1) ∠ABC = ∠DEF = 90°
H(2) AC = DF
S(3) BC = EF
A
B C
D
E F
Then ∆ABC ≅ ∆DEF (by RHS congruence)

PROPERTIESOF TRIANGLE
A
B C
A Triangle in which two sides are equal in length is called ISOSCELESTRIANGLE. So, ∆ABC
is a isosceles triangle with AB = BC.

Angles opposite to equal sides of an isosceles triangle are equal.
B C
A
Here, ∠ABC = ∠ ACB

The sides opposite to equal angles of a triangle are equal.
CB
A
Here, AB = AC

Theorem on inequalities in a triangle
If two sides of a triangle are unequal, the angle opposite to the longer side is larger ( or greater)
10
8
9
Here, by comparing we will get that-
Angle opposite to the longer side(10) is greater(i.e. 90°)

In any triangle, the side opposite to the longer angle is longer.
10
8
9
Here, by comparing we will get that-
Side(i.e. 10) opposite to longer angle (90°) is longer.

The sum of any two side of a triangle is greater than the third side.
10
8
9
Here by comparing we get-
9+8>10
8+10>9
10+9>8
So, sum of any two sides is greater than the third side.

SUMMARY
1.Two figures are congruent, if they are of the same shape and size.
2.If two sides and the included angle of one triangle is equal to the two sides and the included angle then the two triangles are
congruent(by SAS).
3.If two angles and the included side of one triangle are equal to the two angles and the included side of other triangle then the
two triangles are congruent( by ASA).
4.If two angles and the one side of one triangle is equal to the two angles and the corresponding side of other triangle then the
two triangles are congruent(by AAS).
5.If three sides of a triangle is equal to the three sides of other triangle then the two triangles are congruent(by SSS).
6.If in two right-angled triangle, hypotenuse one side of the triangle are equal to the hypotenuse and one side of the other
triangle then the two triangle are congruent.(by RHS)
7.Angles opposite to equal sides of a triangle are equal.
8.Sides opposite to equal angles of a triangle are equal.
9.Each angle of equilateral triangle are 60°
10.In a triangle, angles opposite to the longer side is larger
11.In a triangle, side opposite to the larger angle is longer.
12.Sum of any two sides of triangle is greater than the third side.

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Maths ppt of class 9 CBSE topic Triangles

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Maths ppt of class 9 CBSE topic Triangles