In A Parallelogram, Opposite Sides Are Equal


 
 
Concept Explanation
 

In A Parallelogram, Opposite Sides Are Equal

Theorem 2: In parallelogram, opposite sides are equal.

GIVEN  A parallelogram ABCD

TO PROVE  AB = CD and DA = BC

Construction  Join AC

Proof  Since ABCD is a parallelogram. Therefore, AB large parallel DC and AD large parallel BC

Now, AD large parallel BC and transversal AC intersects them at A and C respectively.

large therefore      <DAC = <BCA        ...(i)       [ Alternate interior angles]

Again, AB large parallel DC and transversal AC intersects them at A and C respectively.

large therefore     <BAC = <DCA       ....(ii)       [Alternate interior angles]

Noe, in large Delta sADC and CBA, we have

        <DAC = <BCA                     [From (i)]

          AC = AC                          [Common side]

and, <DCA = <BAC                     [From (ii)]

So, by ASA-criterion of congruence

   large DeltaADC large cong large DeltaCBA

large Rightarrow  AD = CB and DC = BA                    [large because Corresponding parts of congruent triangles are equal]

Converse Theorem: A quadrilateral is a parallelogram if its opposite sides are equal

Given : A quadrilateral ABCD in which AB= CD and AD= BC

To Prove: ABCD is a Parallelogram.

Const: Join AC

Proof: In Delta ACB; and ;Delta CAD

   AC= AC       [ Common ]

  AB  = CD      [Given]

   BC = AD      [Given]

therefore Delta ACB; cong ;Delta CAD  [ By SSS Criteria of Congruence]

  therefore angle CAB;= angle ACD           [CPCT]

But they are alternate interior angles when AB and CD are straight lines and AC is the transversal. As they are equal therefore AB || CD

and therefore angle ACB;= angle CAD      [CPCT]

But they are alternate interior angles when BC and AD are straight lines and AC is the transversal. As they are equal therefore BC || AD.

As both the opposite pair of sides are parallel therefore ABCD is a parallelogram.

Illustration: Given a triangle ABC lines PQ, QR and PR are drawn from vertex B, A and C such that PQ || AC, QR || BC and PR || AB. Prove that BC is half or QR.

Proof: BC || QR and PR || AB Therefore ABCR is a parallelogram as both the opposite pair of sides are parallel.

BC = AR        ------------(i)     [ Opposite sides of a parallelogram are equal ]

Similarly, BC || QR and PQ || AC Therefore AQBC is a parallelogram as both the opposite pair of sides are parallel.

BC = AQ       --------------(ii)    [ Opposite sides of a parallelogram are equal ]

From (i) and (ii)

BC = AQ= AR

QR = AQ+ AR = BC + BC = 2 BC

BC= frac{1}{2}QR

 

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

In the following figure, which sides of the parallelogram are equal?

Right Option : D
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Explanation
Question : 2

In the following figure, which sides are parallel?

Right Option : C
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Explanation
Question : 3

In the following figure, if PS = QR, PQ = SR, then the quadrilateral PQRS is a ________________.

Right Option : C
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Explanation
 
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