a: \(A=1+3+3^2+...+3^{11}\)
\(=\left(1+3\right)+\left(3^2+3^3\right)+...+\left(3^{10}+3^{11}\right)\)
\(=\left(1+3\right)+3^2\left(1+3\right)+...+3^{10}\left(1+3\right)\)
\(=4\left(1+3^2+...+3^{10}\right)⋮4\)
b: \(B=16^5+2^{15}\)
\(=\left(2^4\right)^5+2^{15}\)
\(=2^{20}+2^{15}=2^{15}\left(2^5+1\right)=2^{15}\cdot33⋮33\)
c: \(C=5+5^2+5^3+...+5^8\)
\(=\left(5+5^2\right)+\left(5^3+5^4\right)+...+\left(5^7+5^8\right)\)
\(=30+5^2\left(5+5^2\right)+...+5^6\left(5+5^2\right)\)
\(=30\left(1+5^2+...+5^6\right)⋮30\)
d: \(45⋮9;99⋮9;180⋮9\)
=>\(45+99+180⋮9\)
=>D chia hết cho 9
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