Câu 3
Đặt \left\{ \begin{array}{l} V_{17}=a\\ V_{20}=b \end{array} \right.
\Rightarrow \left\{ \begin{array}{l} a-b=9\\ a^2+b^2=153 \end{array} \right.
\Rightarrow \left\{ \begin{array}{l} ab=36\\ a+b=15 \end{array} \right.
\Rightarrow \left\{ \begin{array}{l} a=12\\ b=3 \end{array} \right.
\Rightarrow \left\{ \begin{array}{l} V_0.q^{16}=12\\ V_0.q^{19}=3 \end{array} \right.
Tính ra V_0, q là ra S_{11}