\(A=\left(3+3^2\right)+\left(3^3+3^4\right)+...+\left(3^{21}+3^{22}\right)\)
\(A=3.4+3^3.4+....+3^{21}.4\)
\(A=4.\left(3+3^3+...+3^{21}\right)\)
A chia hết cho 4 (đpcm)
A=3+32+33+...+322
A=(3+32)+(33+34)+...+(321+322)
A=3(1+3)+33(1+3)+...+321(1+3)
A=3.4+33.4+...+321.4
A=4(3+33+...+321) chia hết cho 4 - đpcm
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