bc-ab=1/4adbc+ab=1/2ad2ab=1/4adad=8ab=8*4=32bc=1/4ad+ab=1/4*32+4=12
设bc=x则ac=4+xcd=ac=4+xad=ac+cd=8+2x由bc-ab=1/4adx-4=1/4(8+2x)得x=12