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Question

In the quadrilateral given below, AB||DC, E and F are mid point of of non-parallel sides AD and BC respectively. Calculate AB, if DC=8cm and EF=9cm.
1458411_abe4df55a94f479987d5d9c3defe401e.png

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Solution

It is given that
A Quadrilateral which AB||DCAB||DC
EE and FF is the mid point of non-parallel sides ADAD and BCBC
To prove:
(i) EFEF , if AB=6 cmAB=6 cm and DC=4 cmDC=4 cm
(ii) ABAB , if DC=8 cmDC=8 cm and EF=9 cmEF=9 cm
Proof:
We know that
The length of the segment joining the mid point of two non-parallel sides is half sum of the lengths of the parallel sides
EE and FF are the midpoints of ADAD and BCBC,
EF=12(AB+CD).....(1)EF=12(AB+CD).....(1)
AB=6 cmAB=6 cm and DC=4 cmDC=4 cm
Substituting in equation (1)(1)
EF=182(6+4)EF=182(6+4)
By further calculation
EF=12×10=5 cmEF=12×10=5 cm
(ii) DC=8 cmDC=8 cm and EF=9 cmEF=9 cm
Substituting equation (1)
EF=12(AB+DC)EF=12(AB+DC)
By further calculation 9=12(AB+8)9=12(AB+8)
18=AB+818=AB+8
So we get,
188=AB18−8=AB
AB=10 cm

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