Giải hệ [tex]\left\{\begin{matrix} u_{17}-u_{20}=9\\ u_{17}^{2}+u_{20}^{2}=153 \end{matrix}\right.\Rightarrow u_{17}=12, u_{20}=3 , or, u_{17}=-3,u_{20}=-12[/tex]
- TH1: [tex]\left\{\begin{matrix} u_{17}=12\\ u_{20}=3 \end{matrix}\right.\Rightarrow \left\{\begin{matrix} u_1+16d=12\\ u_1+19d=3 \end{matrix}\right.\Leftrightarrow \left\{\begin{matrix} u_1=60\\ d=-3 \end{matrix}\right.[/tex]
- TH2: tương tự