Phương trình ${\text{3 . 2}}^{\text{x}}{\text{ - 4}}^{\text{x - 1}}\text{ - 8 = 0}$ có 2 nghiệm x₁,x₂ và có tổng x₁ + x₂ là:

A. 2

B. 3

${\text{3 . 2}}^{\text{x}}{\text{ - 4}}^{\text{x - 1}}\text{ - 8 = 0}\iff {\text{3 . 2}}^{\text{x}}\text{ - }\frac{{\text{4}}^{\text{x}}}{\text{4}}\text{ - 8 = 0}\iff {\text{3 . 2}}^{\text{x}}\text{ - }\frac{\text{1}}{\text{4}}{\left({\text{2}}^{\text{x}}\right)}^{\text{2}}\text{ - 8 = 0.}$

Đặt ${\text{2}}^{\text{x}}\text{ = t\hspace{0.17em}\hspace{0.17em}}\left(\text{t > 0}\right)$ khi đó phương trình trở thành

$3t-\frac{1}{4}{t}^2-8=0\iff \left[\begin{array}{c}t=8\left(tm\right)\\ t=4\left(tm\right)\end{array}\right.\iff \left[\begin{array}{c}{2}^x=8\\ 2^x=4\end{array}\right.\iff \left[\begin{array}{c}{x}_1=3\\ x_2=2\end{array}\right.\Rightarrow x_1+x_2=5$

Chọn D.