lg(3-x)/(3+x)=lg(1-x) (3-x)/(3+x)=(1-x) (3-x)=(3+x)(1-x) 3-x=3-x+x-x (x-1)x=0 解得x=0或x=1 又lg(1-x)的1-x>0 x<1 所以取x=0