Tìm công sai của cấp số cộng sau:\(\left\{ {\begin{array}{*{20}{c}}{{{\rm{u}}_{\rm{2}}}{\rm{ + }}{{\rm{u}}_{\rm{4}}} - {{\rm{u}}_{\rm{6}}}{\rm{ = }} - {\rm{7}}}\\{{{\rm{u}}_{\rm{8}}} - {{\rm{u}}_{\rm{7}}}{\rm{ = 2}}{{\rm{u}}_{\rm{4}}}}\end{array}} \right.\)

\[\left\{ {\begin{array}{*{20}{c}}{{{\rm{u}}_{\rm{2}}}{\rm{ + }}{{\rm{u}}_{\rm{4}}} - {{\rm{u}}_{\rm{6}}}{\rm{ = }} - {\rm{7}}}\\{{{\rm{u}}_{\rm{8}}} - {{\rm{u}}_{\rm{7}}}{\rm{ = 2}}{{\rm{u}}_{\rm{4}}}}\end{array}} \right. \Leftrightarrow \left\{ {\begin{array}{*{20}{c}}{\left( {{{\rm{u}}_{\rm{1}}}{\rm{ + d}}} \right){\rm{ + }}\left( {{{\rm{u}}_{\rm{1}}}{\rm{ + 3d}}} \right) - \left( {{{\rm{u}}_{\rm{1}}}{\rm{ + 5d}}} \right){\rm{ = }} - {\rm{7}}}\\{\left( {{{\rm{u}}_{\rm{1}}}{\rm{ + 7d}}} \right) - \left( {{{\rm{u}}_{\rm{1}}}{\rm{ + 6d}}} \right){\rm{ = 2}}\left( {{{\rm{u}}_{\rm{1}}}{\rm{ + 3d}}} \right)}\end{array}} \right. \Leftrightarrow \left\{ {\begin{array}{*{20}{c}}{{{\rm{u}}_{\rm{1}}} - {\rm{d = }} - {\rm{7}}}\\{{\rm{2}}{{\rm{u}}_{\rm{1}}}{\rm{ + 5d = 0}}}\end{array}} \right.\]

\[ \Leftrightarrow \left\{ {\begin{array}{*{20}{c}}{{{\rm{u}}_{\rm{1}}}{\rm{ = }} - {\rm{5}}}\\{{\rm{d = 2}}}\end{array}} \right.\]

Đáp án cần chọn là: A