Đáp án:
\(\begin{array}{l}
a.\left\{ \begin{array}{l}
{u_1} = - 2\\
d = 3
\end{array} \right.\\
b.{S_{2019}} = 6107475
\end{array}\)
Giải thích các bước giải:
\(\begin{array}{l}
a.\left\{ \begin{array}{l}
{u_3} + {u_7} = 20\\
{u_4} + {u_{11}} = 35
\end{array} \right. \leftrightarrow \left\{ \begin{array}{l}
{u_1} + 2d + {u_1} + 6d = 20\\
{u_1} + 3d + {u_1} + 10d = 35
\end{array} \right.\\
\leftrightarrow \left\{ \begin{array}{l}
2{u_1} + 8d = 20\\
2{u_1} + 13d = 35
\end{array} \right. \leftrightarrow \left\{ \begin{array}{l}
{u_1} = - 2\\
d = 3
\end{array} \right.
\end{array}\)
\(b.{S_{2019}} = \frac{{(2{u_1} + 2018d).2019}}{2} = 6107475\)
