Chứng tỏ A chia hết cho 35
Biết A= \(2 + 2^2+2^3+2^4+2^5+...+2^{2016}\)
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A=7 mu 2020 mu 2019-3 mu 2016 mu 2015 :5 chung to A la so chan
A=\(\text{2}^{1}+\text{2}^{2}+\text{2}^{3}+\text{2}^{4}+...+\text{2}^{2016}\)
=\((\text{2}^{1}+\text{2}^{2})+(\text{2}^{3}+\text{2}^{4})+...+(\text{2}^{2015}+\text{2}^{2016})\)
=\(2.(1+2)+\text{2}^{3}(1+2)+...+\text{2}^{2015}(1+2)\)
=\((2+\text{2}^{3}+\text{2}^{5}+...+\text{2}^{2015}).(1+2)\)
=\((2+\text{2}^{3}+\text{2}^{5}+...+\text{2}^{2015}).3\)⋮\(3\)
Vậy A⋮3
A=\(\text{2}^{1}+\text{2}^{2}+\text{2}^{3}+\text{2}^{4}+...+\text{2}^{2016}\)
=\((\text{2}^{1}+\text{2}^{2}+\text{2}^{3})+(\text{2}^{4}+\text{2}^{5}+\text{2}^{6})+...+(\text{2}^{2014}+\text{2}^{2015}+\text{2}^{2016})\)
=\(2(1+\text{2}^{1}+\text{2}^{2})+\text{2}^{4}(1+\text{2}^{1}+\text{2}^{2})+...+\text{2}^{2014}(1+\text{2}^{1}+\text{2}^{2})\)
=\((2+\text{2}^{4}+...+\text{2}^{2014})(1+\text{2}^{1}+\text{2}^{2})\)
=\((2+\text{2}^{4}+...+\text{2}^{2014})7\)⋮\(7\)
Vậy A⋮7
Ta có:A=(2+22+26)+(23+24+28)+...+(22011+22012+22016) (Có 672 cặp)
A=2.(1+2+32)+23.(1+2+32)+...+22011.(1+2+32)
A=2.35+23.35+...+22011.35
A=35.(2+23+...+22011) chia hết cho 35
Vậy A chia hết cho 35
Dễ VCL