c.
[tex]PT\Leftrightarrow (sin^3x+2)^3=-54+81sinx\\\Leftrightarrow sin^9x+6sin^6x+12sin^3x+8=-54+81sinx\\sinx=t\epsilon [-1;1]\\\Leftrightarrow t^9+6t^6+12t^3-81t+62=0\\\Leftrightarrow (t-1)^2(t+2)(t^6+3t^4+4t^3+9t^2+6t+31)[/tex]
[tex]t=-2[/tex] (Loại)
[tex]t^6+3t^4+4t^3+9t^2+6t+31\\=t^6+3t^4+9t^2+4(t^3+1)+6(t+1)+21>0 \forall t\epsilon [-1;1][/tex]
PT $sinx=1 \\ \Leftrightarrow...$
d.
[tex]DK:x\geq \frac{-2}{3}[/tex]
[tex]PT\Leftrightarrow cot^4x-4cot^2x+4=27cotx+18\\cotx=t\\PT\Leftrightarrow t^4-4t^2-27cotx-14=0\\\Leftrightarrow (t^2+3t+7)(t^2-3t-2)=0\\\Leftrightarrow t^2-3t-2=0(\Delta _{t^2+3t+7}<0)\\ \Leftrightarrow...[/tex]
[TẶNG BẠN] TRỌN BỘ Bí kíp học tốt 08 môn
