Answers

2016-01-17T14:34:43+05:30
You just add the ar(AOB) on both sides to get ar(ABD) = ar(ACB). 
then by theorem 
triangles on the same base and having equal area lie between same parallels. thus, CD is parallel to AB. 
Hence, ABCD is a trapezium 
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2016-01-19T16:34:49+05:30
Please see the attached image For solution :
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