In quadrilateral ABCD, AB is parallel to CD ∠ DAB = ∠ DCB, ab = 4, BD = 5 ABC:S Quadrilateral ABCD=

Because AB / / CD,
So the angle DAB + angle ADC = 180 degrees, (two lines are parallel, and the internal angles on the same side are complementary)
Because angle DAB = angle DCB,
So angle DCB + angle ADC = 180 degrees, (equivalent substitution)
So ad / / BC, (the same side angle complements each other, two lines are parallel)
So the quadrilateral ABCD is a parallelogram
So the area of triangle ABC: the area of quadrilateral ABCD = 1:2

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