\(A=1+3+3^2+.....+3^{11}\)
\(A=\left(1+3+3^2\right)+....+\left(3^9+3^{10}+3^{11}\right)\)
\(A=\left(3^0.1+3^0.3+3^0.3^2\right)+....+\left(3^9.1+3^9.3+3^9.3^2\right)\)
\(A=1.\left(1+3+3^2\right)+....+3^9\left(1+3+3^2\right)\)
\(A=1.13+....+3^9.13\)
\(A=13.\left(1+....+3^9\right)⋮13\left(đpcm\right)\)
\(A=1+3+3^2+3^3+...+3^{11}\)
\(=\left(1+3+3^2\right)+\left(3^3+3^4+3^5\right)+...+\left(3^9+3^{10}+3^{11}\right)\)
\(=13+13.3^3+...+13.3^9\)
\(=13.\left(1+3^3+...+3^9\right)⋮13\)
\(\Rightarrow\)\(đpcm\)
\(B=2^{11}+2^{12}+2^{13}+...+2^{21}+2^{22}\)
\(=\left(2^{11}+2^{12}+2^{13}+2^{14}+2^{15}+2^{16}\right)+\left(2^{17}+2^{18}+2^{19}+2^{20}+2^{21}+2^{22}\right)\)
\(=2^{11}\left(1+2+4+8+16+32\right)+2^{17}\left(1+2+4+8+16+32\right)\)
\(=2^{11}.63+2^{17}.63=63\left(2^{11}+2^{17}\right)=21.3.\left(2^{11}+2^{17}\right)⋮21\left(đpcm\right)\)
ngủ đi mai dậy học tiếp
A=1+3+3^2+....+3^10+3^11
=(1+3+3^2)+....+(3^9+3^10+3^11)
=13+13.3^3+....+13.3^9
=13(3^3+3^4+....+3^9)chia hết cho 13
Vậy......