Câu hỏi
Cho khai triển nhị thức New-ton: \({{\left( x-2 \right)}^{2018}}={{a}_{0}}+{{a}_{1}}x+{{a}_{2}}{{x}^{2}}+......+{{a}_{2018}}{{x}^{2018}}\) Tính tổng \(S={{a}_{0}}-{{a}_{1}}+{{a}_{2}}-{{a}_{3}}+....+{{\left( -1 \right)}^{k}}{{a}_{k}}+....+{{a}_{2018}}?\)
- A \(S=0\)
- B \(S={{3}^{2018}}\)
- C \(S=-{{3}^{2018}}\)
- D \(S=1\)
Phương pháp giải:
Áp dụng khai triển nhị thức New-ton: \({{\left( a+b \right)}^{n}}=\sum\limits_{k=0}^{n}{C_{n}^{k}{{a}^{n-k}}{{b}^{k}}.}\)
Lời giải chi tiết:
Ta có: \({{\left( x-2 \right)}^{2018}}={{\left( 2-x \right)}^{2018}}=\sum\limits_{k=0}^{2018}{C_{2018}^{k}}{{\left( -x \right)}^{2018-k}}{{2}^{k}}=C_{2018}^{0}{{.2}^{2018}}-C_{2018}^{1}{{.2}^{2017}}x+C_{2018}^{2}{{.2}^{2016}}{{x}^{2}}-....+C_{2018}^{2018}{{x}^{2018}}\)
Với \(x=-1\) ta có: \({{\left( 2+1 \right)}^{2018}}=C_{2018}^{0}{{.2}^{2018}}+C_{2018}^{1}{{.2}^{2017}}+C_{2018}^{2}{{.2}^{2016}}+....+C_{2018}^{2018}={{3}^{2018}}\)
\(\begin{align} & \Rightarrow S={{a}_{0}}-{{a}_{1}}+{{a}_{2}}-{{a}_{3}}+....+{{\left( -1 \right)}^{k}}{{a}_{k}}+....+{{a}_{2018}} \\ & \ \ \ \ \ \ \ ={{2}^{2018}}C_{2018}^{0}+{{2}^{2017}}C_{2018}^{1}+{{2}^{2016}}C_{2018}^{2}+........+C_{2018}^{2018}={{3}^{2018}}. \\\end{align}\)
Chọn B.