Nine Chapters on Mathematical Art, Vol. 1

Now there is a trapezoidal field xie tian literally "sloped field", one side 30 bu wide, the other side 42 bu wide, and 64 bu long.
Ask: What is the area of the field?
Answer: 9 mu and 144 bu
There is another trapezoidal field, 65 bu wide, with one edge 100 bu long and one edge 72 bu long.
Ask: What is the area of the field?
Answer: 23 mu and 70 bu
Procedure: Combine the two sloped sides and halve them, then multiply by the length. Or, one can halve the length, or the width, and multiply by the combined sides.
Divide by the mu-method.
Combining and halving serves to compensate for the deficiency with the excess.