Quadrilateral With One Opposite Pair Of Side Equal and Parallel is a Parallelogram


 
 
Concept Explanation
 

Quadrilateral With One Opposite Pair Of Side Equal and Parallel is a Parallelogram

Quadrilateral With One Opposite Pair of Side Equal and Parallel is a Parallelogram:

Theorem: A quadrilateral is a parallelogram, if its one pair of opposite sides are equal and parallel

Given: A quadrilateral ABCD such  that AB = CD and AB || CD.

To Prove: ABCD is a parallelogram

Construction: Join AC

Proof: In Delta ABC and Delta CDA

        AB = CD                    [Given]

  angle BAC = angle ACD     [ Alternate angles are equal when AB || CD and AC is the transversal]

       AC = CA                    [Common]

Delta ABC cong Delta CDA       [SAS Criteria of Congruence]

    angle BCA = angle DAC     [C.P.C.T.}

But they are Alternate angles when BC and AD are straight lines and AC is the transversal, as they are equal so BC || AD

Now both the opposite pair of sides are parallel

So, ABCD is a parallelogram

Illustration: In a triangle ABC median AO is extended to D such that AO = OD. Prove that BACD is a parallelogram

Solution:   In Delta BAO and Delta CDO

     BO = OC    [ AO is the median]

     angle BOA = angle COD       [ Vertically opposite angle ]

    AO = OD      [Given]

therefore Delta BOA = Delta COD      [ SAS criteria of congruence]

  Rightarrow  AB = CD   [CPCT}

Rightarrow angle ABO = angle DCO      [CPCT]

But they are Alternate angles when AB and CD are straight lines and BC is the transversal, as they are equal so AB || CD.

Now AB = CD and AB || CD,

Therefore BACD is a parallelogram as its one pair of opposite sides are equal and parallel

 
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