f(x) = (3^x-1)/(3^x+1) 定义域为实数集R.记 u = (3^x - 1)/(3^x + 1),u不等于1.[ 若u=1,则 3^x -1 = 3^x +1,-1 = 1.矛盾,因此,u不等于1]当x > 0时,0 < 3^x - 1 < 3^x + 1,0 < u < 1.当x < 0时,u = [1 - 3^(-x)]/[1+...