Question

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Answer 1

HI there !

To prove : AB || CD

In ΔDCO ,

OC = OD

We know that , in an isosceles triangle , where two sides of the triangle are equal , the angles opposite to it are also equal.
so ,
∠DCO = ∠CDO [ angles opposite to equal sides ]         ---->(1)

we know that 

∠BAO = ∠DCO

so , from (1)

∠BAO = ∠CDO

∠BAO and ∠CDO form the alternate interior , within the lines AB and CD , with BC is the transversal.

∠BAO = ∠CDO [ alternate interior angles  ]

hence , AB || CD