Đáp án:
Giải thích các bước giải:
Bài 2:
`a, A = 1 + 4 + 4^2 + 4^3 +......+ 4^2012`
`A = ( 1 + 4 + 4^2 ) + ( 4^3 + 4^4 + 4^5 ) +......+ ( 4^2010 + 4^2011 + 4^2012 )`
`A = ( 1 + 4 + 16 ) + 4^3 . ( 1 + 4 + 16 ) +....+ 4^2010 . ( 1 + 4 + 16 )`
`A = 21 . ( 1 + 4^3 +......+ 4^2010 ) vdots 21`
`-> A vdots 21`
`b, B = 2 + 2^2 + 2^3 +.......+ 2^100`
`B = ( 2 + 2^2 + 2^3 + 2^4 + 2^5 ) +.......+ ( 2^96 + 2^97 + 2^98 + 2^99 + 2^100 )`
`B = 2 . ( 1 + 2 + 4 + 8 + 16 ) +......+ 2^96 . ( 1 + 2 + 4 + 8 + 16 )`
`B = 31 . ( 2 +.......+ 2^96 ) vdots 31`
`-> B vdots 31`
`#haducdung2009`
