1
A=2+22+23+....+210
=> A=(2+22+23+24)+.....+(27+28+29+210)
=>A=30.1+....+27.(2+22+23+24)
=> A=30.1+...+27.30
=>A=30.(1+...+27)
Vì A⋮30 mà 30⋮5=>A⋮5
2.
A=1+4+42+43+....+498
=>A=(1+42+43+44+45)+....+(493+494+495+496+497+498)
=>A=1365.1+....+493.(1+42+43+44+45)
=>A=1365.1+...+493.1365
=>A=1365.(1+...+493)
Vì A⋮1365 mà 1365⋮91=>A⋮91
3.
A=3n+3+2n+3+3n+1+2n+2
=>A=3n.33+2n.23+3n.3+2n.22
=>A=3n.27+2n.8+3n.3+2n.4
=>A=3n.(27+3)+2n.(8+4)
=>A=3n.30+2n.12
Vì 30⋮6 và 12⋮6
=>A⋮6