giúp mink vs ạ

Câu 4:
$\displaystyle \begin{cases}
u_{1} \ +\ u_{3} \ =\ 80\\
u_{2} \ +\ u_{4} \ =\ 400
\end{cases}$ nên $\displaystyle \begin{cases}
u_{1} \ +\ u_{3} \ =\ 80\\
u_{1} .q\ +\ u_{3} .q\ =\ 400
\end{cases}$ nên $\displaystyle \begin{cases}
u_{1} \ +\ u_{3} \ =\ 80\\
( u_{1} +\ u_{3}) .q\ =\ 400
\end{cases}$
Suy ra $\displaystyle \frac{1}{q} \ =\ \frac{80}{400} \ =\ \frac{1}{5}$ nên $\displaystyle q\ =\ 5$
Ta có: $\displaystyle u_{1} \ +\ u_{3} \ =\ 80$
nên $\displaystyle u_{1} \ +\ u_{1} q^{2} \ =\ 80$
nên $\displaystyle u_{1} \ =\ \frac{80}{1\ +\ q^{2}} \ \ =\ \frac{40}{13}$