If a parallelogram have the same base with a triangle and in the same parlallels, the parallelogram is double of the triangle.
For let the parallelogram ABCD have the same base BC with the triangle EBC,
and let it be in the same parallels BC, AE;
I say that the parallelogram ABCD is the double of the triangle BEC.
For let AC be joined.
Then triangle ABC is equal to the triangle EBC;
for it is on the same base BC with it and in the same parallels BC, AE.
But parallelogram ABCD is double of triangle ABC; for the diameter AB bisects
it;
so that parallelogram ABCD is also double of the triangle EBC.
Therefore etc.
Q.E.D