Cho cấp số cộng ( u n ) biết u6 = 48 và u11 = 83 . Tìm cặp ( u1 ; d ) .

20/39

Cho cấp số cộng \(\left( {{u_n}} \right)\)biết \({u_6} = 48\) và \({u_{11}} = 83\). Tìm cặp \(\left( {{u_1};{\rm{ }}d} \right)\).

\(\left( {7;13} \right)\).

\(\left( { - 7; - 13} \right)\).

\(\left( {13;{\rm{ }}7} \right)\).

\(\left( { - 13;{\rm{ }} - 7} \right)\).

Giải thích

Chọn C

\(\left\{ \begin{array}{l}{u_6} = 48\\{u_{11}} = 83\end{array} \right. \Leftrightarrow \left\{ \begin{array}{l}{u_1} + 5d = 48\\{u_1} + 10d = 83\end{array} \right. \Leftrightarrow \left\{ \begin{array}{l}{u_1} = 13\\d = 7\end{array} \right.\).