CHÚC BẠN HỌC TỐT!!!!
a, `B=3.(1+3+3^2+....+3^{119})`
`=>B` $\vdots$ `3`
b, `B= 3+3^2+3^3+...+3^120`
`B=(3+3^2)+(3^3+3^4)+....+(3^{119}+3^{120})`
`B=3.(1+3)+3^3(1+3)+....+3^{119}.(1+3)`
`B=3.4+3^3.4+......+3^{119}.4`
`B=4(3+3^3+.....+3^{119})`
`=>B` $\vdots$ `4`
c,`B = 3+3^2+3^3+...+3^120`
`B=(3+3^2+3^3)+(3^4+3^5+3^6)+.....+(3^{118}+3^{119}+3^{120})`
`B=3(1+3+9)+3^4(1+3+9)+.......+3^{118}(1+3+9)`
`B=3.13+3^4.13+......+3^{118}.13`
`B=13.(3+3^4+.....+3^{118})`
`=>B` $\vdots$ `13`
CHÚC BẠN HỌC TỐT!!!!
