Giải thích các bước giải:
a. \(U_{3}=U_{1}.q^{3-1}=2.(\frac{-1}{2})^{2}=\frac{1}{2}\)
\(U_{5}=U_{1}.q^{5-1}=2.(\frac{-1}{2})^{4}=\frac{1}{8}\)
b. \(U_{n}=U_{1}.q^{n-1}\)
\( \Leftrightarrow \frac{1}{128}=2.(\frac{-1}{2})^{n-1}\)
\( \Leftrightarrow \frac{1}{128}=2.(\frac{\frac{-1}{2})^{n}}{(\frac{-1}{2})}\)
\( \Leftrightarrow (\frac{-1}{2})^{n}=\frac{-1}{512}\)
\( \Leftrightarrow n=9\)
c. \(S_{4}=\frac{U_{1}.(1-q^{n})}{1-q}=\frac{2.(1-(-\frac{1}{2})^{4})}{1-(\frac{-1}{2})}=\frac{5}{4}\)
