AB=AD*2/(2+3+4)=AD*2/9,[AB为全长的9分之2]
BC=AD*3/9,
CD=AD*4/9,
MB+BC+CN=MN
AB/2+BC+CD/2=18,
(AD*2/9)/2+AD*3/9+(AD*4/9)/2=18,
AD*2+AD*3*2+AD*4=18*9*2,
12*AD=18*18,
AD=18*18/12=27(cm)
AB=AD*2/(2+3+4)=AD*2/9,[AB为全长的9分之2]
BC=AD*3/9,
CD=AD*4/9,
MB+BC+CN=MN
AB/2+BC+CD/2=18,
(AD*2/9)/2+AD*3/9+(AD*4/9)/2=18,
AD*2+AD*3*2+AD*4=18*9*2,
12*AD=18*18,
AD=18*18/12=27(cm)