Answer:
Step-by-step explanation:
Because if you put together two trapezoids, one in a reversed position, you get a paralelogram. There the side of the parallelogram is the sum of the trapezoid bases, as in the picture.
The are of paralelogram is: A=(b+B)×h
For trapezoid, half of parallelogram:
A1=A/2=(b+B)×h/2
