Opposite Angles of a Parallelogram are Equal


 
 
Concept Explanation
 

Opposite Angles of a Parallelogram are Equal

Theorem 3: The opposite angles of a parallelogram are equal.

GIVEN  A parallelogram ABCD

To prove  <A = <C and <B = <D

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

Now, AB large parallel DC and transversal AD intersects them at A and D respectively.

large therefore   <A + <D = 180  ....(i)   [ large because Sum of consecutive interior angles is 180]

Again, AD large parallel BC and DC intersects them at D and C respectively.

large therefore   <C + <D = 180  ...(ii)     [ large because Sum of consecutive interior angles is 180]

From (i) and (ii), we get

      <A + <D = <D + <C

large Rightarrow  <A = <C

Similarly, <B = <D.

Hence, <A = <C and <B = <D

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

Given : A quadrilateral ABCD in whichangle A = angle C and angle B = angle D

To Prove: ABCD is a Parallelogram.

Proof: In a quadrilateral ABCD

    angle A = angle C                .......(1)      [Given]

    angle B = angle D              .......(2)      [Given]

Adding (1) and (2)

angle A +angle B = angle C +angle D           .............(3)

Now angle A +angle B + angle C +angle D= 360^0           [Angle Sum Property]

Using equation (3) we get

Rightarrow ;;;angle A +angle B + angle A +angle B= 360^0

Rightarrow ;;;2(angle A +angle B)= 360^0

Rightarrow ;;;angle A +angle B= 180^0

Rightarrow ;;;angle A +angle B=angle C +angle D= 180^0

But  angle A and  angle B are cointerior angles when AD and BC are straight lines and AB is the transversal cutting them

As their sum is 180^0  Therefore AD || BC         .......(4)

Again ;;;angle A +angle B= 180^0 and  angle A = angle C is given

Rightarrow ;;;angle C +angle B= 180^0

But  angle C and  angle B are cointerior angles when AB and CD are straight lines and BC is the transversal cutting them

As their sum is 180^0  Therefore AB || CD                 .......(5)

From (4) and (5)

AD || BC and  AB || CD

Hence ABCD is a a parallelogram

 

Illustration: Find all the angles of the parallelogram ABCD the figure given.

Solution: In triangle BCD

angle B +angle C+angle D= 180^0            {angle sum property of triangle.]

5x+ 20+ 2x+ 10 +3x= 180

10x+ 30 = 180

10x = 180-30=150

x= 15

angle B = 5x+ 20 = 5(15)+20= 75+ 20= 95^0

angle C = 2x+ 10 = 2(15)+10= 30+ 10= 40^0

angle D = 3x = 3(15)= 45^0

In a parallelogram sum of adjacent angles= 180

angle D + angle C= 180^0

angle D +40^0= 180^0Rightarrow angle D = 140^0

In a parallelogram opposite angles are equal

angle A = angle C = 40^0     and angle B = angle D = 140^0

Hence angle A = 40^0 , angle B = 140^0, angle C = 40^0 ; and ;angle D = 140^0

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

In the following figure, ABFE is a parallelogram. Which of the following is true about the angles of this parallelogram?

Right Option : C
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Explanation
Question : 2

Which of the following is not true about the following figure?

Right Option : C
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Explanation
Question : 3

Which of the following is true about the following figure?

Right Option : C
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Explanation
 
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