Giải thích các bước giải:
\(a.\left\{ \begin{array}{l}
{u_8} - {u_6} = 10\\
{u_8} + {u_3} = 20
\end{array} \right. \leftrightarrow \left\{ \begin{array}{l}
{u_1} + 7d - {u_1} - 5d = 10\\
{u_1} + 7d + {u_1} + 2d = 20
\end{array} \right. \leftrightarrow \left\{ \begin{array}{l}
2d = 10\\
2{u_1} + 9d = 20
\end{array} \right. \Leftrightarrow \left\{ \begin{array}{l}
d = 5\\
{u_1} = \frac{{ - 25}}{2}
\end{array} \right.\)
\(b.\left\{ \begin{array}{l}
2{u_2} - {u_3} = 10\\
{u_7} + {u_4} = 10
\end{array} \right. \leftrightarrow \left\{ \begin{array}{l}
2{u_1} + 2d - {u_1} - 2d = 10\\
{u_1} + 6d + {u_1} + 3d = 10
\end{array} \right. \leftrightarrow \left\{ \begin{array}{l}
{u_1} = 10\\
2{u_1} + 9d = 10
\end{array} \right. \Leftrightarrow \left\{ \begin{array}{l}
d = \frac{{ - 10}}{9}\\
{u_1} = 10
\end{array} \right.\)
\(c.\left\{ \begin{array}{l}
{u_{10}} = 20\\
{S_{10}} = 30
\end{array} \right. \leftrightarrow \left\{ \begin{array}{l}
{u_1} + 9d = 10\\
\frac{{(2{u_1} + 9d).10}}{2} = 30
\end{array} \right. \leftrightarrow \left\{ \begin{array}{l}
{u_1} + 9d = 10\\
2{u_1} + 9d = 6
\end{array} \right. \leftrightarrow \left\{ \begin{array}{l}
{u_1} = - 4\\
d = \frac{{14}}{9}
\end{array} \right.\)
