1.a.Ta có: $\left\{ \begin{array}{l} U_9=U_1+8d\\U_2=U_1+d\\U_{13}=U_1+12d\\U_6=U_1+5d \end{array} \right.$Suy ra: $\left\{ \begin{array}{l} U_1+8d=5(U_1+d)\\U_1+12d=2(U_1+5d)+5\end{array} \right.\Leftrightarrow \left\{ \begin{array}{l} U_1=3\\ d=4 \end{array} \right.$Từ đó: $U_{20}=U_1+19d=79;U_{30}=U_1+29d=119$Suy ra: $S_{30}=\frac{30}{2}(U_1+U_{30})=1830$
1.a.Ta có: $\left\{ \begin{array}{l} U_9=U_1+8d\\U_2=U_1+d\\U_{13}=U_1+12d\\U_6=U_1+5d \end{array} \right.$Suy ra: $\left\{ \begin{array}{l} U_1+8d=5(U_1+d)\\U_1+12d=2(U_1+5d)+5\end{array} \right.\Leftrightarrow \left\{ \begin{array}{l} U_1=3\\ d=4 \end{array} \right.$Từ đó: $U_{20}=U_1+19d=79;U_{30}=U_1+29d=119$
1.a.Ta có: $\left\{ \begin{array}{l} U_9=U_1+8d\\U_2=U_1+d\\U_{13}=U_1+12d\\U_6=U_1+5d \end{array} \right.$Suy ra: $\left\{ \begin{array}{l} U_1+8d=5(U_1+d)\\U_1+12d=2(U_1+5d)+5\end{array} \right.\Leftrightarrow \left\{ \begin{array}{l} U_1=3\\ d=4 \end{array} \right.$Từ đó: $U_{20}=U_1+19d=79;U_{30}=U_1+29d=119$
Suy ra: $S_{30}=\frac{30}{2}(U_1+U_{30})=1830$