Cho cấp số cộng (u_n) có: \[{{\rm{u}}_{\rm{1}}}{\rm{ = }} - {\rm{0,1;}}\,\,{\rm{d = 1}}\]. Khẳng định nào sau đây là đúng?

A. \[{{\rm{S}}_{\rm{n}}}{\rm{ = n}}{{\rm{u}}_{\rm{1}}}{\rm{ + }}\frac{{{\rm{n}}\left( {{\rm{n}} - {\rm{1}}} \right){\rm{d}}}}{{\rm{2}}}\]

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

C. \[{{\rm{S}}_{\rm{n}}}{\rm{ = }}\frac{{{\rm{n}}{{\rm{u}}_{\rm{1}}}{\rm{ + n}}\left( {{\rm{n}} - {\rm{1}}} \right){\rm{d}}}}{{\rm{2}}}\]

D. \[{{\rm{S}}_{\rm{n}}}{\rm{ = n}}{{\rm{u}}_{\rm{1}}}{\rm{ + }}\frac{{{{\rm{n}}^{\rm{2}}}{\rm{d}}}}{{\rm{2}}}\]

A. 5.

B. −5.

C. 1.

D. −1.

A. d = 3

B. d = 1

C. d = – 3

D. d = – 1

A. Số thứ 25.

B. Số thứ 26.

C. Số thứ 35.

D. Số thứ 36.

A. n = 20.

B. n = 21.

C. n = 22.

D. n = 23.

A. \[{{\rm{a}}^{\rm{2}}}{\rm{ + }}{{\rm{c}}^{\rm{2}}}{\rm{ = 2ab}} - {\rm{2bc}}\]

B. \[{{\rm{a}}^{\rm{2}}}{\rm{ + }}{{\rm{c}}^{\rm{2}}}{\rm{ + 2ac = 4}}{{\rm{b}}^{\rm{2}}}\]

C.\[{{\rm{a}}^{\rm{2}}} - {{\rm{c}}^{\rm{2}}}{\rm{ = ab}} - {\rm{bc}}\]

D. \[{{\rm{a}}^{\rm{2}}} - {{\rm{c}}^{\rm{2}}}{\rm{ = 2ab}} - {\rm{2bc}}\]

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