Đặt `f(x) = u`

`u' = -3x^{2} + 6x;` `u' = 0 <=> [(x = 2),(x = 0):}`

BBT:

$\color{#FA8072}{\text{___}}$ $\color{#F28D84}{\text{___}}$ $\color{#EFAE98}{\text{___}}$ $\color{#E7C2AB}{\text{___}}$ $\color{#DFD6BF}{\text{___}}$ $\color{#D7E9D3}{\text{___}}$ $\color{#CFFDD7}{\text{___}}$ $\color{#C7FBEA}{\text{___}}$ $\color{#BFF9FE}{\text{___}}$ $\color{#B7F8F7}{\text{___}}$ $\color{#AFF6EC}{\text{___}}$ $\color{#A7F5DF}{\text{___}}$ $\color{#9FF3D2}{\text{___}}$ $\color{#97F2C5}{\text{___}}$ $\color{#8FF1B9}{\text{___}}$ $\color{#87F0AC}{\text{___}}$ $\color{#7FEFA0}{\text{___}}$ $\color{#77ED93}{\text{___}}$

(Hình ảnh)

$\color{#FA8072}{\text{___}}$ $\color{#F28D84}{\text{___}}$ $\color{#EFAE98}{\text{___}}$ $\color{#E7C2AB}{\text{___}}$ $\color{#DFD6BF}{\text{___}}$ $\color{#D7E9D3}{\text{___}}$ $\color{#CFFDD7}{\text{___}}$ $\color{#C7FBEA}{\text{___}}$ $\color{#BFF9FE}{\text{___}}$ $\color{#B7F8F7}{\text{___}}$ $\color{#AFF6EC}{\text{___}}$ $\color{#A7F5DF}{\text{___}}$ $\color{#9FF3D2}{\text{___}}$ $\color{#97F2C5}{\text{___}}$ $\color{#8FF1B9}{\text{___}}$ $\color{#87F0AC}{\text{___}}$ $\color{#7FEFA0}{\text{___}}$ $\color{#77ED93}{\text{___}}$

Vậy phương trình `f(f(x)) = 4` có `1` nghiệm

$\color{#FA8072}{\text{$\textit{$\circ$ hungnguyen4269}$}}$