Hey Guys,
Just wanted to weigh in on this. This particular geometry issue IS tested on the GMAT. It's actually just the rule that equal sides are opposite equal angles.
Because Triangle BCD is an equilateral triangle, all the sides are the same (BD=BC=CD). Next, because Angle A = Angle B, AD = BD, which means AD = DC.
That means the two triangles (ABD and BDC) have the same base (AD and DC) and the same height. So their areas are equal.
-t
P.S. To see the actual GMAT play with this trick, see Question 123 from the second edition of the Quant Official Guide.
Tommy Wallach, Company Expert
ManhattanGMAT
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