Ta có:M=23+24+25+...+28+29
=(23+24)+(25+26)+..+(28+29)
=23(1+2)+25(1+2)+...+28(1+2)
=23.3+25.3+...+28.3
=3(23+25+...+28)
\(\Rightarrow\)M\(⋮\)3
Vậy.....
A=\(2\left(1+2\right)+2^3\left(1+2\right)+...+2^8\left(1+2\right)\)
\(=2.3+2^3.3+...+2^8.3\)
\(=3\left(2+2^3+...+2^8\right)\)chia hết cho 3
=>\(A\)chia hết cho 3