1 câu trả lời
Đáp án+Giải thích các bước giải:
$\left\{\begin{array}{l} -8u_1+u_4=-168 \\ 4u_1+2u_2+u_3=-84\end{array} \right.\\ \Leftrightarrow \left\{\begin{array}{l} -8u_1+u_1q^3=-168 \\ 4u_1+2u_1q+u_1q^2=-84\end{array} \right.\\ \Rightarrow \dfrac{-8u_1+u_1q^3}{4u_1+2u_1q+u_1q^2}=\dfrac{-168}{-84}\\ \Leftrightarrow \dfrac{-8+q^3}{4+2q+q^2}=2\\ \Rightarrow 8+4q+2q^2=-8+q^3\\ \Leftrightarrow q^3-2q^2-4q-16=0\\ \Leftrightarrow q=4\\ -8u_1+u_4=-168\\ \Leftrightarrow -8u_1+u_1q^3=-168\\ \Leftrightarrow u_1(q^3-8)=-168\\ \Leftrightarrow u_1=-\dfrac{168}{q^3-8}=-3\\ S_{20}=\dfrac{u_1(1-q^{20})}{1-q}=\dfrac{-3(1-4^{20})}{1-4}=1-4^{20}.$
