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Q.

In the following figure, ABCD is a trapezium in which AB = 7 cm, AD = BC = 5 cm, DC = x cm, and distance between AB and DC is 4 cm. Find the value of x and among the following choices, the area of trapezium ABCD is:


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a

20 c m 2

b

30 c m 2

c

40 c m 2

d

50 c m 2  

answer is C.

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Detailed Solution

Given, AB=7cm, AD=5cm, BC=5cm
Distance between AB and DC is 4 cm.                              Draw a line,
AL perpendicular to DC and BM perpendicular to MC .
Question Image Therefore, AL=4 cm, BM=4 cm and LM = 7 cm.
Consider ΔADL , where AL perpendicular to DC .
By Pythagoras theorem,
A D 2 =A L 2 +D L 2   5 2 = 4 2 +D L 2   D L 2 =2516   DL= 9   DL=3cm
Consider ΔBMC , where BM perpendicular to MC .
By Pythagoras theorem,
B C 2 =B M 2 +M C 2   5 2 = 4 2 +M C 2   M C 2 =2516   MC= 9   MC=3cm
Since, AL and BM are perpendicular on CD,
MC = LD = 3 cm
And, LM = AB = 7 cm.
Calculate the value of x ,  x=DL+LM+MC Substitute the values of DL=3cm, LM=7cm and MC=3cm , we get,
x=DL+LM+MC   x=3+7+3   x=13cm
Calculate the area of trapezium ABCD .
Area of the trapezium = 1 2 (sum of the bases)×height
Area of trapezium ABCD= 1 2 (AB+DC)×AL   = 1 2 (7+13)×4   =20×2   =40 c m 2
The area of the trapezium ABCD is 40c m 2 .
Hence option 3) is correct.
 
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