Ta có: \(u_n=u_1q^{n-1}\)
a) Nếu \(\left\{ \matrix{q > 0 \hfill \cr {u_1} < 0 \hfill \cr} \right. \Rightarrow {u_n} < 0,\forall n\)
b) Nếu \(\left\{ \matrix{q < 0 \hfill \cr {u_1} < 0 \hfill \cr} \right.\) \(\Rightarrow u_n< 0\) khi \(n\) lẻ và \(u_n> 0\) khi \(n \) chẵn.
