2f(x)-f(-x)=lg(x+1).(1)
2f(-x)-f(x)=lg(-x+1).(2)
2×(1)+(2)得3f(x)=2lg(x+1)+lg(-x+1)=lg[(1+x)²(1-x)]
故f(x)=(1/3)lg[(1+x)²(1-x)]