If , and the area of the parallelogram , determine the area of .
If a triangle and a parallelogram share a common base lying on one of the two parallel lines including them, then the area of the triangle is equal to half the area of the parallelogram.
Therefore, the area of is half the area of
parallelogram , and the area of is half the area of parallelogram .
Since and together form polygon
and parallelograms and together form parallelogram , the area of is equal to half of parallelogram .