Google
Câu hỏi: Cho cấp số nhân $\left( {{u}_{n}} \right)$ có ${{u}_{6}}=27$, công bội $q=\dfrac{1}{3}$. Số hạng ${{u}_{3}}$ bằng
A. $81$.
B. $243$.
C. $27$.
D. $729$.
A. $81$.
B. $243$.
C. $27$.
D. $729$.
${{u}_{3}}={{u}_{1}}.{{q}^{2}}$
${{u}_{6}}={{u}_{1}}.{{q}^{5}}={{u}_{1}}.{{q}^{2}}.{{q}^{3}}={{u}_{3}}.{{q}^{3}}\Rightarrow {{u}_{3}}=\dfrac{{{u}_{6}}}{{{q}^{3}}}=\dfrac{27}{{{\left( \dfrac{1}{3} \right)}^{3}}}=729.$
${{u}_{6}}={{u}_{1}}.{{q}^{5}}={{u}_{1}}.{{q}^{2}}.{{q}^{3}}={{u}_{3}}.{{q}^{3}}\Rightarrow {{u}_{3}}=\dfrac{{{u}_{6}}}{{{q}^{3}}}=\dfrac{27}{{{\left( \dfrac{1}{3} \right)}^{3}}}=729.$
Đáp án D.