Câu 23.
Ta có: \(n\ge2\)
\(u_n=u_1\cdot q^{n-1}\)\(\Rightarrow u_2=u_1\cdot q^{2-1}\)
\(\Rightarrow6=2\cdot q\Rightarrow q=3\)
Chọn B
Câu 24.
Có \(n\ge2\)
\(u_n=u_1\cdot q^{n-1}\Rightarrow u_4=u_1\cdot q^{n-1}\)
\(\Rightarrow1=u_1\cdot q^{4-1}=u_1\cdot3^{4-1}\)
\(\Rightarrow u_1=\dfrac{1}{27}\)
Chọn B
\(u_1=2\\ u_2=6\\ Mà:u_2=u_1.q\\ \Leftrightarrow6=2.q\\ \Leftrightarrow q=3\\ \Rightarrow B\)
Câu 24:
\(u_4=1\\ q=3\\ ADCT:u_n=u_1.q^{n-1}\left(n\ge2\right)\\ \Rightarrow u_4=u_1.q^3\\ \Leftrightarrow1=u_1.3^3\\ \Leftrightarrow u_1=\dfrac{1}{3^3}=\dfrac{1}{27}\)
=>Chọn B
