Cho cấp số cộng (un) biết: { u 9 = 5 u 2 u 13 = 2 u 6 + 5 . Chọn đáp án đúng.

\(\left\{ {\begin{array}{*{20}{c}}{{{\rm{u}}_{\rm{9}}}{\rm{ = 5}}{{\rm{u}}_{\rm{2}}}}\\{{{\rm{u}}_{{\rm{13}}}}{\rm{ = 2}}{{\rm{u}}_{\rm{6}}}{\rm{ + 5}}}\end{array}} \right. \Leftrightarrow \left\{ {\begin{array}{*{20}{c}}{\left( {{u_1} + 8d} \right){\rm{ = }}5\left( {{u_1} + d} \right)}\\{\left( {{u_1} + 12d} \right){\rm{ = }}2\left( {{u_1} + 5d} \right) + 5}\end{array}} \right. \Leftrightarrow \left\{ {\begin{array}{*{20}{c}}{{\rm{4}}{{\rm{u}}_{\rm{1}}} - {\rm{3d = 0}}}\\{{{\rm{u}}_{\rm{1}}} - {\rm{2d = }} - {\rm{5}}}\end{array}} \right. \Leftrightarrow \left\{ {\begin{array}{*{20}{c}}{{{\rm{u}}_{\rm{1}}}{\rm{ = }}3}\\{{\rm{d = 4}}}\end{array}} \right.\)

Đáp án cần chọn là: A