I know this....,
ΔABC and Δ ADC are on the same base AB (and on the opposite sides of AB....)
In ΔDAC, AO is the median, (given, CD is bisected by AB)
ar(DAO) = ar(CAO) -----------{1} ( a median of a triangle divides it into 2 triangles of equal areas)
Similarlly, BO is the median of ΔCBD
∴ ar(DBO) = ar(CBO) -----------{2} ( a median of a triangle divides it into 2 triangles of equal areas)
Adding {1} and {2}
ar(DAO) + ar(DBO) = ar(CAO) + ar(CBO)
i.e., ar(ABD) = ar(ABC)
or, ar(ABC) = ar(ABD)
HENCE PROVED
Hope this Helps......
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Cheers!!