Đáp án + Giải thích các bước giải:
`a)`
`S=3+3^{2}+3^{3}+...+3^{1998}`
`=(3+3^{2}+3^{3})+(3^{4}+3^{5}+3^{6})+....+(3^{1996}+3^{1997}+3^{1998})`
`=(3+3^{2}+3^{3})+3^{3}(3+3^{2}+3^{3})+....+3^{1995}(3+3^{2}+3^{3})`
`=39+3^{3}.39+....+3^{1995}.39`
`=39.(1+3^{3}+....+3^{1995})\vdots 39`
`b)`
`P=3+3^{2}+3^{3}+....+3^{1000}`
`=(3+3^{2}+3^{3}+3^{4})+(3^{5}+3^{6}+3^{7}+3^{8})+.....+(3^{997}+3^{998}+3^{999}+3^{1000})`
`=(3+3^{2}+3^{3}+3^{4})+3^{4}(3+3^{2}+3^{3}+3^{4})+.....+3^{996}(3+3^{2}+3^{3}+3^{4})`
`=120+3^{4}.120+....+3^{996}.120`
`=120.(1+3^{4}+....+3^{996})\vdots 120`
