Câu hỏi 16

\(C_n^{n - 2} + 2n = 9\)

\( \Leftrightarrow \dfrac{{n!}}{{\left( {n - 2} \right)!.2!}} + 2n - 9 = 0\)

\( \Leftrightarrow \dfrac{{\left( {n - 1} \right)n}}{2} + 2n - 9 = 0\) 

\( \Leftrightarrow {n^2} - n + 4n - 18 = 0\)

\( \Leftrightarrow {n^2} + 3n - 18 = 0\)

\( \Leftrightarrow \left[ \begin{array}{l}n = 3\,\,\,\,\,\,\left( {tm} \right)\\n =  - 6\,\,\left( {ktm} \right)\end{array} \right.\) 

Chọn A.