因为BC=1/3AB=1/4CD 所以AB=3/4CD
因为MN=BM+CN-2BC
=1/2AB+1/2CD-2BC
=1/2(AB+CD)-2BC
=1/2(3/4CD+CD)-2*1/4CD
=7/8CD-4/8CD
=3/8CD=18
所以CD=48 AB=3/4*48=36 BC=1/4*48=12
因为BC=1/3AB=1/4CD 所以AB=3/4CD
因为MN=BM+CN-2BC
=1/2AB+1/2CD-2BC
=1/2(AB+CD)-2BC
=1/2(3/4CD+CD)-2*1/4CD
=7/8CD-4/8CD
=3/8CD=18
所以CD=48 AB=3/4*48=36 BC=1/4*48=12