\(B=5+5^2+5^3+...+5^{96}\)
\(B=\left(5+5^2\right)+\left(5^3+5^4\right)+...+\left(5^{95}+5^{96}\right)\)
\(B=5\cdot\left(1+5\right)+5^3\cdot\left(1+5\right)+5^{95}\cdot\left(1+5\right)\)
\(B=6\cdot\left(5+5^3+...5^{95}\right)\)
B chia hết cho 6
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\(B=5+5^2+5^3+...+5^{96}\)
\(B=\left(5+5^2+5^3\right)+\left(5^4+5^5+5^6\right)+....+\left(5^{94}+5^{95}+5^{96}\right)\)
\(B=5\cdot\left(1+5+25\right)+5^4\cdot\left(1+5+25\right)+...+5^{94}\cdot\left(1+5+25\right)\)
\(B=31\cdot\left(5+5^4+...+5^{94}\right)\)
B chia hết cho 31
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\(B=5+5^2+5^3+...+5^{96}\)
\(B=\left(5+5^3\right)+\left(5^2+5^4\right)+...+\left(5^{94}+5^{96}\right)\)
\(B=5\cdot\left(1+25\right)+5^2\cdot\left(1+25\right)+...+5^{94}\cdot\left(1+25\right)\)
\(B=26\cdot\left(5+5^2+...+5^{94}\right)\)
B chia hết cho 26
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\(B=5+5^2+5^3+...+5^{96}\)
\(B=\left(5+5^4\right)+\left(5^2+5^5\right)+...+\left(5^{93}+5^{96}\right)\)
\(B=5\cdot\left(1+125\right)+5^2\cdot\left(1+125\right)+...+5^{93}\cdot\left(1+125\right)\)
\(B=126\cdot\left(5+5^2+...+5^{93}\right)\)
B chia hết cho 126