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10.Sa se arate ca numarul S=3+3^2+3^3+...+3^222 este divizibil cu 12

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[tex]\bf S=3+3^{2}+3^{3}+......+3^{222}[/tex]

  • Grupam termenii

[tex]\bf S=(3^{1}+3^{2})+3^{2}\cdot(3^{3-2}+3^{4-2})+......+3^{220}\cdot(3^{221-220}+3^{222-220})[/tex]

[tex]\bf S=(3+9)+3^{2} \cdot (3^{1}+3^{2})+......+3^{220}\cdot(3^{1}+3^{2})[/tex]

[tex]\bf S=12+3^{2} \cdot (3+9)+......+3^{220}\cdot(3+9)[/tex]

[tex]\bf S=12+3^{2} \cdot 12+......+3^{220}\cdot 12[/tex]

  • Dam factor comun 12

[tex]\boxed{\boxed{\bf S=12\cdot (1 +3^{2}+3^{3}+ ......3^{219}+3^{220})~\vdots ~12}}[/tex]

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