如图做辅助线ae.
(1)e是cd的中点,得ce=ed
(2)△bce和△aed等低同高,∴S△bce=S△aed=1cm²
(3)△bce与△abe等低,且高是△abe的1/2倍 ∴S△abe=2cm²
S平行四边形abcd=1+1+2=4cm²