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Câu hỏi: Cho cấp số nhân $\left( {{u}_{n}} \right)$ với ${{u}_{4}}=1$ ; $q=3$. Tìm ${{u}_{1}}$ ?
A. ${{u}_{1}}=\dfrac{1}{9}$.
B. ${{u}_{1}}=9$.
C. ${{u}_{1}}=27$.
D. ${{u}_{1}}=\dfrac{1}{27}$.
Ta có: ${{u}_{4}}={{u}_{1}}.{{q}^{3}}\Rightarrow {{u}_{1}}=\dfrac{{{u}_{4}}}{{{q}^{3}}}=\dfrac{1}{{{3}^{3}}}=\dfrac{1}{27}$.
A. ${{u}_{1}}=\dfrac{1}{9}$.
B. ${{u}_{1}}=9$.
C. ${{u}_{1}}=27$.
D. ${{u}_{1}}=\dfrac{1}{27}$.
Ta có: ${{u}_{4}}={{u}_{1}}.{{q}^{3}}\Rightarrow {{u}_{1}}=\dfrac{{{u}_{4}}}{{{q}^{3}}}=\dfrac{1}{{{3}^{3}}}=\dfrac{1}{27}$.
Đáp án D.