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Câu hỏi: Cho cấp số nhân $\left( {{u}_{n}} \right)$ có ${{u}_{1}}=1, {{u}_{2}}=-2$. Giá trị của ${{u}_{2019}}$ bằng
A. ${{u}_{2019}}=-{{2}^{2018}}$.
B. ${{u}_{2019}}={{2}^{2018}}$.
C. ${{u}_{2019}}=-{{2}^{2019}}$.
D. ${{u}_{2019}}={{2}^{2019}}$.
A. ${{u}_{2019}}=-{{2}^{2018}}$.
B. ${{u}_{2019}}={{2}^{2018}}$.
C. ${{u}_{2019}}=-{{2}^{2019}}$.
D. ${{u}_{2019}}={{2}^{2019}}$.
Ta có: $q=\dfrac{{{u}_{2}}}{{{u}_{1}}}=\dfrac{-2}{1}=-2$.
${{u}_{n}}={{u}_{1}}.{{q}^{n-1}}\Leftrightarrow {{u}_{2019}}={{u}_{1}}.{{q}^{2018}}=1.{{\left( -2 \right)}^{2018}}={{2}^{2018}}$.
${{u}_{n}}={{u}_{1}}.{{q}^{n-1}}\Leftrightarrow {{u}_{2019}}={{u}_{1}}.{{q}^{2018}}=1.{{\left( -2 \right)}^{2018}}={{2}^{2018}}$.
Đáp án B.