Congruence of Triangles By SAS Criteria


 
 
Concept Explanation
 

Congruence of Triangles By SAS Criteria

Theorem: Two triangles are congruent if two sides and the included angle of one are equal to the corresponding sides and the included angle of the other triangle.

Given:  Two triangles ABC and DEF such that large AB=DE, AC=DF;and;angle A=angle D

To Prove:  large Delta ABCcong Delta DEF

Proof: Place large Delta ABC;over; Delta DEF such that vertex A falls on vertex D and the side AB falls on side DE,

As AB = DE      [Given]

So vertex B falls on vertex E.

Since angle A=angle D. Therefore, AC will fall on DF.

But AC = DF       [ Given]

Therefore, C will fall on F.

Thus, AC coincides with DF.

Now, B falls on E and C falls on F. Therefore, BC coincides with EF.

Thus, large Delta ABC will coincide with large Delta DEF.

Hence, by definition of congruence,large Delta ABCcong Delta DEF

 
Illustration: In large Delta ABC and large Delta PQR, AB = PQ, BC = QR and CB and RQ are extended to X and Y respectively and large angle ABX=angle PQY. Prove that large Delta ABCcong Delta PQR.

Solution:    We have,    large angle ABX=angle PQY

large Rightarrow ;;;180^{circ}-angle ABC=180^{circ}-angle PQR   

                                       [ large because angle ABX+angle ABC=180     large therefore angle ABX=180^{circ}-angle ABC   Similarly, large angle POY=180^{circ}-angle PQR ]

large Rightarrow ;;;-angle ABC=-angle PQR

large Rightarrow ;;;angle ABC=angle PQR                                    .....(i)

In large Delta ABC and large Delta PQR, we have

          AB = PQ                               [Given]

       large angle ABC=angle PQR      [From (i)]

and,     BC = QR                              [Given]

So, by SAS criterion of congruence, we have

         large Delta ABCcong Delta PQR                    

Illustration: In the given figure, AC = AE, AB = AD and angle BAD=angle EAC.Show that BC = DE

Solution: We have,

  angle BAD=angle EAC                                                                            [Given]

Rightarrow  angle BAD+angle DAC=angle EAC + angle DAC                             [Adding  angle DAC to both sides]

Rightarrow   angle BAC=angle DAE

Now, in triangles ABC and ADE,

   AB=AD                                                                                            [Given]

  angle BAC=angle DAE                                                                              [From (i)]

and,  AC=AE                                                                                     [Given]

So, by SAS congruence criterion, we have

  Delta ABCcong Delta ADE               

Rightarrow  BC=DE                                                                                     [CPCT]

Sample Questions
(More Questions for each concept available in Login)
Question : 1

In congruent triangles ABC and DEF, ∠A = ∠E = 40∘, and ∠F = 65∘, then ∠B =

Right Option : C
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Explanation
Question : 2

In the given figure, AB = AC. If angle ACD=125^{0}, then measure of angle A is

Right Option : B
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Explanation
Question : 3

In the figure shown above, if the two triangles are congruent by SAS criteria then which of the following conditions are not true?

Right Option : C
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Explanation
 
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