Question

giúp em với ạ

Answer 1

Chọn A

Answer 2

2.

\(u_2=u_1.q\)

\(\Leftrightarrow q=\dfrac{u_2}{u_1}=2\)

Answer 3

3.

\(\left\{{}\begin{matrix}u_2=3\\u_4=27\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}u_1.q=3\\u_1.q^3=27\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}u_1.q=3\\3q^2=27\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}u_1.q=3\\q=\pm3\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}u_1=1\\q=3\end{matrix}\right.\\\left\{{}\begin{matrix}u_1=-1\\q=-3\end{matrix}\right.\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}u_3=9\\u_3=-9\end{matrix}\right.\)