$$\eqalign{
& a)\,\,y = 2{x^3} + 6x\,\,\left( {D = R} \right) \cr
& y' = 6{x^2} + 6 = 6\left( {{x^2} + 1} \right) > 0\,\,\forall x \in R \cr
& \Rightarrow Ham\,\,so\,DB/R \cr
& \Rightarrow Ham\,\,so\,\,khong\,\,co\,\,cuc\,\,tri. \cr
& b)\,\,y = {x^4} - 4{x^2} \cr
& y' = 4{x^3} - 8x = 0 \cr
& \Leftrightarrow 4x\left( {{x^2} - 2} \right) = 0 \Leftrightarrow \left[ \matrix{
x = 0 \hfill \cr
x = \sqrt 2 \hfill \cr
x = - \sqrt 2 \hfill \cr} \right. \cr
& BXD\,\,y': \cr
& - \infty \,\,\,\,\, - \,\,\,\,\,\, - \sqrt 2 \,\,\,\,\,\, + \,\,\,\,\,\,\,0\,\,\,\,\,\,\,\,\,\,\,\, - \,\,\,\,\,\,\,\,\,\,\,\sqrt 2 \,\,\,\,\,\,\,\, + \,\,\,\,\,\,\,\,\, + \infty \cr
& \Rightarrow Ham\,\,so\,\,DB/\left( { - \sqrt 2 ;0} \right);\,\,\left( {\sqrt 2 ; + \infty } \right) \cr
& \,\,\,\,\,\,Ham\,\,so\,\,NB/\left( { - \infty ; - \sqrt 2 } \right);\left( {0;\sqrt 2 } \right) \cr
& \Rightarrow {x_{CD}} = 0;\,\,{x_{CT}} = \pm \sqrt 2 \cr
& c)\,\,y = - {x^4} - 4{x^2} \cr
& y' = - 4{x^3} - 8x \cr
& \,\,\,\,\,\, = - 4x\left( {{x^2} + 2} \right) = 0 \Leftrightarrow x = 0 \cr
& BXD\,\,y': \cr
& - \infty \,\,\,\,\,\, + \,\,\,\,\,\,\,0\,\,\,\,\,\, - \,\,\,\,\,\,\,\,\, + \infty \cr
& \Rightarrow Ham\,\,\,so\,DB/\left( { - \infty ;0} \right),\,\,NB/\left( {0; + \infty } \right) \cr
& \Rightarrow {x_{CD}} = 0 \cr} $$
