a,
`S`=1`+`3`+`3^2`+`3^3`+...+`3^99`
`3S =3+3^2+3^3+3^4+....+3^100+3^101`
`3S-S= 2S= 3^101 - 1`
S = `(3^101-1)/2`
b,
`S` =`1`+`3`+`3^2`+`3^3`+...+`3^98`+`3^99`
`S= 1.(1+3)+3^2.(1+3)+....+3^98.(1+3)`
`S= 1 . 4 + 3^2 . 4 +....+ 3^98. 4`
`S= 4 . (1+3^2+...+3^98)`
⇒ S $\vdots$ 4