过P点做底边AD的平行线,交AB与Q
梯形中位线PQ=1/2(AD+BC)
S△PAB=S△PQA+S△PQB=1/2*PQ*(1/2*梯形高)+1/2*PQ*(1/2*梯形高)
=1/2 * PQ*梯形高
=1/2 * S梯形ABCD