Answer:
S2 = (S1+S3)/2
Step-by-step explanation:
The area of a trapezoid is the product of the midline length and the height. The parallel lines have divided the height into three equal pieces, so the midline of the middle piece will be the average of the midline lengths of the top and bottom pieces.
The result is that the area of the middle part is the average of the areas of the top and bottom parts:
S2 = (S1+S3)/2