21. 设 y = (1/x)^tanx, 则 lny = - tanx lnx = - lnx/cotx lim lny = - lim lnx/cotx (∞/∞) = - lim (1/x)/[-(cscx)^2] = lim (sinx)^2/x = 0 则 原式 = lim y = 1
