A = 3 + `3^2` + `3^3` + . . . + `3^2010`
A = ( 3 + `3^2` + `3^3` ) + . . . + ( `3^2008` + `3^2009` + `3^2010` )
A = 3 ( 1 + 3 + `3^2` ) + . . . + `3^2008` ( 1 + 3 + `3^2` )
A = 3 . 13 + `3^4` . 13 + . . . + `3^2008` . 13
A = 13 . ( 3 + `3^4` + . . . + `3^2008` )
Mà 13 chia hết cho 13 `=>` A chia hết cho 13 ( đpcm )