Cho cấp số nhân $\left(u_{n}\right)$ với $u_{1}=3, u_{2}=-6$. Tính $u_{9}$.

<p><span class="math-tex">$\begin{array}{l}u_{1}=3, u_{2}=-6 \Rightarrow q=-2 \\ u_{9}=u_{1} q^{8}=3 \cdot(-2)^{8}=768\end{array}$</span></p>

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Accepted Answer

$\begin{array}{l}u_{1}=3, u_{2}=-6 \Rightarrow q=-2 \ u_{9}=u_{1} q^{8}=3 \cdot(-2)^{8}=768\end{array}$