Câu hỏi 32

\(C_n^0 - 2C_n^1 + {2^2}C_n^2 - ... + {\left( { - 1} \right)^n}{.2^n}.C_n^n\)

\( + )\) Xét: \({\left( {x - 2} \right)^n} = C_n^0.{x^n}.{\left( { - 2} \right)^0} + C_n^1.{x^{n - 1}}.{\left( { - 2} \right)^1} + C_n^2.{x^{n - 2}}.{\left( { - 2} \right)^2} + ... + C_n^n.{x^0}.{\left( { - 2} \right)^n}\)

\( + )\)Thay \(x = 1\) vào cả 2 vế:

\({\left( {1 - 2} \right)^n} = C_n^0 - 2C_n^1 + {2^2}.C_n^2 - ... + {\left( { - 1} \right)^n}{.2^n}.C_n^n\)\( \Leftrightarrow S = {\left( { - 1} \right)^n}\)

Chọn C.