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Mình nhầm \(C^1_{2016}a_{2015}\)thành \(C^1_{2016}a^{2015}\)
Gọi \(A=C_{2016}^0+C_{2016}^1+C_{2016}^2+...+C_{2016}^{2016}\)
\(=2^{2016}\) (HỆ QUẢ CỦA NHỊ THỨC NIUTON)
\(\Rightarrow\) \(S=2015+\left(A-C_{2016}^0-C_{2016}^1\right)\)
\(=2015+2^{2016}-1-2016\)
\(=2^{2016}-2\)
\(=\lim\limits_{x\rightarrow0}\dfrac{\left(x^2+2016\right)\left(\sqrt[3]{1+3x}-1\right)+x^2}{x}=\lim\limits_{x\rightarrow0}\dfrac{\dfrac{3x\left(x^2+2016\right)}{\sqrt[3]{\left(1+3x\right)^2}+\sqrt[3]{1+3x}+1}+x^2}{x}\)
\(=\lim\limits_{x\rightarrow0}\left(\dfrac{3\left(x^2+2016\right)}{\sqrt[3]{\left(1+3x\right)^2}+\sqrt[3]{1+3x}+1}+x\right)=\dfrac{3.2016}{3}=2016\)
\(S=\dfrac{1}{2018!\left(2019-2018\right)!}+\dfrac{1}{2016!\left(2019-2016\right)!}+...+\dfrac{1}{2!\left(2019-2\right)!}+\dfrac{1}{0!\left(2019-0!\right)}\)
\(\Rightarrow2019!.S=\dfrac{2019!}{2018!\left(2019-2018\right)!}+\dfrac{2019!}{2016!\left(2019-2016\right)!}+...+\dfrac{2019!}{2!\left(2019-2\right)!}+\dfrac{2019!}{0!\left(2019-0\right)!}\)
\(=C_{2019}^{2018}+C_{2019}^{2016}+...+C_{2019}^2+C_{2019}^0\)
\(=\dfrac{1}{2}\left(C_{2019}^0+C_{2019}^1+...+C_{2019}^{2018}+C_{2019}^{2019}\right)\)
\(=\dfrac{1}{2}.2^{2019}=2^{2018}\)
\(\Rightarrow S=\dfrac{2^{2018}}{2019!}\)
Ta có:
\(A=1+2016+2016^2+...+2016^{2016}\)
\(\Rightarrow2016A=2016.\left(1+2016+2016^2+...+2016^{2016}\right)\)
\(=2016+2016^2+2016^3...+2016^{2017}\)
\(\Rightarrow2016A-A=\left(2016+2016^2+2016^3...+2016^{2017}\right)-\left(1+2016+2016^2+...+2016^{2016}\right)\)
\(\Rightarrow2015A=2016^{2017}-1\)
\(\Rightarrow A=\frac{2016^{2017}-1}{2015}\)
Vậy \(A=\frac{2016^{2017}-1}{2015}\)