`A=3^99 -3^98+ 3^97 -3^96+...+3^3 -3^2+3-1`
`=>3A=(3^100+ 3^98+...+3^4+3^2)-(3^99+ 3^97+...+3^3+3)`
`=>3A-A=3.(3^98+ 3^96+...+3^2)-(3^98+ 3^96+...+3^2)`
`3A=(3-1).(3^98+ 3^96+...+3^2)`
`3A=2.(3^98+ 3^96+...+3^2)`
`=>3A=2(3^100+ 3^98+...+3^2)`
`=>A=(2.(3^99+ 3^97+...+3).3)/3`
`=>A=2.(3^99 + 3^97+...+3)`
`=>A=2.[(3^99+3^97)+...+(3^3+3)]`
`=>A=2.[3^97.(3^2+1)+...+3(3^2+1)]`
`=>A=2.(3^97.10+...+3.10)`
`=>A=2.10.(3^97+...+3)`
`=>A=20.(3^97+ 3^95+...+3)`
Vì `20 vdots 4=>20.(3^97 + 3^95+...+3) vdots 4=>A vdots 4`
