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\(A=\frac{1}{1.2.3}+\frac{1}{2.3.4}+...+\frac{1}{18.19.20}=\frac{1}{2}\left(\frac{2}{1.2.3}+\frac{2}{2.3.4}+...+\frac{2}{18.19.20}\right)\)
\(=\frac{1}{2}\left(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...+\frac{1}{18.19}-\frac{1}{19.20}\right)=\frac{1}{2}\left(\frac{1}{1.2}-\frac{1}{19.20}\right)\)
\(=\frac{1}{2}\left(\frac{1}{2}-\frac{1}{380}\right)=\frac{1}{4}-\frac{1}{760}< \frac{1}{4}\)(ĐPCM)
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\(A=1+6+6^2+...+6^9\)
\(=1+\left(6+6^2+6^3\right)+\left(6^4+6^5+6^6\right)+\left(6^7+6^8+6^9\right)\)
\(=1+6\left(1+6+6^2\right)+6^4\left(1+6+6^2\right)+6^7\left(1+6+6^2\right)\)
\(=1+\left(1+6+6^2\right)\left(6+6^4+6^7\right)\)
\(=1+43\left(6+6^4+6^7\right)\)
Ta thấy \(43\left(6+6^4+6^7\right)⋮43\)
nên A chia 43 dư 1
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là sao , yêu cầu là j z