a) \(C=5+5^2+5^3+...+5^{20}=5\left(1+5+5^2+...+5^{19}\right)⋮5\)
b) \(C=5+5^2+5^3+...+5^{20}=5\left(1+5\right)+5^3\left(1+5\right)+...+5^{19}\left(1+5\right)\)
\(=5.6+5^3.6+...+5^{19}.6=6\left(5+5^3+...+5^{19}\right)⋮6\)
c) \(C=5+5^2+5^3+...+5^{20}\)
\(=\left(5+5^2+5^3+5^4\right)+...+\left(5^{17}+5^{18}+5^{19}+5^{20}\right)\)
\(=5\left(1+5+5^2+5^3\right)+...+5^{17}\left(1+5+5^2+5^3\right)\)
\(=5.156+...+5^{17}.156=156\left(5+5^5+...+5^{17}\right)=13.12\left(5+5^5+...+5^{17}\right)⋮13\)
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