6 用罗必塔法则得 原式 = lim(secx)^2/[3(sec3x)^2] = lim(cos3x)^2/[3(cosx)^2] = lim6cos3x(-sin3x)/[6cosx(-sinx)] = limsin6x/sin2x = lim6cos6x/)2cos2x = 3
