本质上是证明两侧的四边形面积的平均值等于中间四边形面积

If AB is the ground, and dot E,F,C's height above AB is h1, h2, h3, respectively, then h1+h3=2*h2.

So, AEG+HCB=2*GFH

Similarly, if DC is ground, we can get: ADE+HFC=2*GEF

So, ADEG+HFCB=2*GEFH