f(x)=(log(2)x-1)/(log(2)x+1)=1-2/(log(2)x+1)
因为x大于1,所以log(2)x>0 =>log(2)x+1>1
=>-2/(log(2)x+1)<-2 =>1-2/(log(2)x+1)<-1
=>方程f(x)=a有解,则a<-1