Cho cấp số cộng (({u_n})) thỏa mãn :(left{ {begin{array}{*{20}{c}}{{u_2} – {u_3} + {u_5} = 10}\{{u_4} + {u_6} = 26}end{array}} right.). Xác định công sai ?

Ta có

(begin{array}{c}left{ {begin{array}{*{20}{c}}{{u_2} – {u_3} + {u_5} = 10}\{{u_4} + {u_6} = 26}end{array}} right. \ Leftrightarrow  left{ begin{array}{l}{u_1} + d – {u_1} – 2d + {u_1} + 4d = 10\{u_1} + 3d + {u_1} + 5d = 26end{array} right.\  Leftrightarrow  left{ begin{array}{l}{u_1} + 3d = 10\2{u_1} + 8d = 26end{array} right.\ Leftrightarrow  left{ begin{array}{l}{u_1} = 1\d = 3end{array} right.end{array})

Chọn A.