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Câu hỏi: Cho cấp số nhân có ${{u}_{1}}=-3$, $q=\dfrac{2}{3}$. Tính ${{u}_{5}}?$
A. ${{u}_{5}}=\dfrac{-27}{16}.$
B. ${{u}_{5}}=\dfrac{-16}{27}.$
C. ${{u}_{5}}=\dfrac{16}{27}.$
D. ${{u}_{5}}=\dfrac{27}{16}.$
A. ${{u}_{5}}=\dfrac{-27}{16}.$
B. ${{u}_{5}}=\dfrac{-16}{27}.$
C. ${{u}_{5}}=\dfrac{16}{27}.$
D. ${{u}_{5}}=\dfrac{27}{16}.$
Ta có: ${{u}_{5}}={{u}_{1}}.{{q}^{4}}=\left( -3 \right){{\left( \dfrac{2}{3} \right)}^{4}}=-\dfrac{16}{27}.$
Đáp án B.