(1)作CE⊥AB于E,DF⊥AB于F,则CE∥DF,
∵S△ABC=S△ABD,
∴ AB•CE= AB•DF,CE=DF.
∴四边形CDFE为矩形,AB∥CD
(2)连接MF、NE.
∴S△MEF= ME•OE= k;S△NEF= NF•OF= k,
∴S△MEF=S△NEF,
∴MN∥EF.
(1)作CE⊥AB于E,DF⊥AB于F,则CE∥DF,
∵S△ABC=S△ABD,
∴ AB•CE= AB•DF,CE=DF.
∴四边形CDFE为矩形,AB∥CD
(2)连接MF、NE.
∴S△MEF= ME•OE= k;S△NEF= NF•OF= k,
∴S△MEF=S△NEF,
∴MN∥EF.