`a)S=1+3+3^3+3^3+...+3^99`
`S=(1+3+3^3+3^3)+(3^4+3^5+3^6+3^7)...+(3^96+ 3^97+ 3^98+ 3^99)`
`S=40.1+40.(3^4)...+40.(3^96)`
`S=40.(1+3^4+...+3^96)`
Vì `40 vdots 40=>40.(1+3^4+...+3^36)=>S vdots 40`
`b)S=1+4+4^2+4^3+...+4^62`
`S=(1+4+4^2)+(4^3+4^4+4^5)+...+(4^60+4^61+4^62)`
`S=21.1+21.4^4+...+21.4^61`
`S=21.(1+4^4+...+4^61)`
Vì `21 vdots 21=>21.(1+4^4+4^61) vdots 21=>S vdots 21`