$$\eqalign{
& y = {1 \over 3}{x^3} - \left( {m + 1} \right){x^2} + \left( {2m + 1} \right)x - {4 \over 3} \cr
& y' = {x^2} - 2\left( {m + 1} \right)x + 2m + 1 \cr
& \Delta ' = {\left( {m + 1} \right)^2} - 2m - 1 = {m^2} \cr
& Ham\,\,so\,\,co\,\,cuc\,\,tri \cr
& \Rightarrow Ham\,\,so\,\,co\,\,2\,\,cuc\,\,tri \cr
& \Rightarrow \Delta ' > 0 \Leftrightarrow m \ne 0 \cr
& \left[ \matrix{
{x_1} = m + 1 + m = 2m + 1 \Rightarrow y = - {4 \over 3}{m^3} + m - 1 \hfill \cr
{x_2} = m + 1 - m = 1 \Rightarrow y = m - 1 \hfill \cr} \right. \cr
& Ham\,so\,\,co\,\,cuc\,\,tri\,\, \in Ox \cr
& \Rightarrow \left[ \matrix{
m - 1 = 0 \hfill \cr
- {4 \over 3}{m^3} + m - 1 = 0 \hfill \cr} \right. \Leftrightarrow \left[ \matrix{
m = 1 \hfill \cr
m \approx - 1,17\,\,\left( {loai} \right) \hfill \cr} \right. \cr
& Vay\,\,m = 1. \cr
& Chon\,\,B \cr} $$
