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

A. \[{{\rm{u}}_{\rm{n}}}{\rm{ = }}{{\rm{u}}_{\rm{1}}}{\rm{.}}{{\rm{d}}^{\rm{n}}}\]

B. \[{{\rm{u}}_{\rm{n}}}{\rm{ = }}{{\rm{u}}_{\rm{1}}}{\rm{.}}{{\rm{d}}^{{\rm{n}} - {\rm{1}}}}\]

C. \[{{\rm{u}}_{\rm{n}}}{\rm{ = }}{{\rm{u}}_{\rm{1}}}{\rm{ + nd}}\]

D. \[{{\rm{u}}_{\rm{n}}}{\rm{ = }}{{\rm{u}}_{\rm{1}}}{\rm{ + }}\left( {{\rm{n}} - {\rm{1}}} \right){\rm{d}}\]

A. −115.

B. −130.

C. 115.

D. 130.

A. \[{{\rm{u}}_{{\rm{15}}}}{\rm{ = 34}}\]

B. \[{{\rm{u}}_{{\rm{15}}}}{\rm{ = 37}}\]

C. \[{{\rm{u}}_{{\rm{13}}}}{\rm{ = 34}}\]

D. \[{{\rm{u}}_{{\rm{13}}}}{\rm{ = 37}}\]

A. P = 79 .

B. P = 16.

C. P = 80.

D. P = 64.

A. 26^o.

B. 162^o.

C. 60^o.

D. 126^o.

A. \(\left\{ {\begin{array}{*{20}{c}}{{{\rm{u}}_{\rm{1}}}{\rm{ = 1}}}\\{{u_{{\rm{n + 1}}}}{\rm{ = }}{{\rm{u}}_{\rm{n}}}{\rm{ + 1}},n \ge 1}\end{array}} \right.\)

B. \(\left\{ {\begin{array}{*{20}{c}}{{{\rm{u}}_{\rm{1}}}{\rm{ = }} - {\rm{1}}}\\{{u_{{\rm{n + 1}}}}{\rm{ = }} - 3{u_n},n \ge 1}\end{array}} \right.\)

C. \(\left\{ {\begin{array}{*{20}{c}}{{{\rm{u}}_{\rm{1}}}{\rm{ = }} - 2}\\{{u_{{\rm{n + 1}}}}{\rm{ = }}2{u_n} + 3,n \ge 1}\end{array}} \right.\)

D. \(\left\{ {\begin{array}{*{20}{c}}{{{\rm{u}}_{\rm{1}}}{\rm{ = }}\frac{\pi }{2}}\\{{u_{{\rm{n + 1}}}}{\rm{ = }}\sin \left( {\frac{\pi }{{\pi - 1}}} \right),n \ge 1}\end{array}} \right.\)

A. 2

B. \(\frac{1}{2}\)

C.\(\frac{2}{3}\)

D. 1