Ta có : (5+5x2+5x3+..+5x4+..+5x60 )
=5x(1+2+...+60)
=5x[(60+1)x60:2]
=5x61x30=5x61x5x6=>chia hết cho 6
\(5+5^2+5^3+5^4+...+5^{60}\)
\(=5.\left(5+1\right)+5^3.\left(5+1\right)+....+5^{49}.\left(5+1\right)\)
\(=5.6+5^3.6+...+5^{49}.6\)
=> \(⋮6\)
\(5+5^2+5^3+...+5^{60}\)
\(=\left(5+5^2+5^3\right)+\left(5^4+5^5+5^6\right)+...+\left(5^{58}+5^{59}+5^{60}\right)\)
\(=5.31+5^4.31+...+5^{58}.31\)
\(\Rightarrow⋮31\)
\(5+5^2+5^3+...+5^{60}\)
\(=\left(5+5^2\right)+\left(5^3+5^4\right)+...+\left(5^{59}+5^{60}\right)\)
\(=5\left(1+5\right)+5^3\left(1+5\right)+...+5^{59}\left(1+5\right)\)
\(=5.6+5^3.6+...+5^{59}.6\)
\(=6\left(5+5^3+...+5^{59}\right)⋮6\)
\(5+5^2+5^3+...+5^{60}\)
\(=\left(5+5^2+5^3\right)+\left(5^4+5^5+5^6\right)+...+\left(5^{58}+5^{59}+5^{60}\right)\)
\(=5\left(1+5+5^2\right)+5^4\left(1+5+5^2\right)+...+5^{58}\left(1+5+5^2\right)\)
\(=5.31+5^4.31+...+5^{58}.31\)
\(=31\left(5+5^4+...+5^{58}\right)⋮31\)
