令(y+z)/x=(x+z)/y=(x+y)/z=k则y+z=kx①，x+z=ky②①+②：y+x+2z=kx+kyx+y+2z=k(x+y)2z=k(x+y) - (x+y)2z=(x+y)(k-1)，则(x+y)/z=2/(k-1)∵(x+y)/z=k∴k=2/(k-1)，则k(k-1)=2k²-k-2=(k+1)(k-2)=0∴k=-1或k=2∴x/(y+z)=1/[(y+z)/x]=1/k=-1或1/2选E
