Đáp án:
Câu `4:`
`a,` `3^2021-3^2019+3^2017`
`=3^2017. 3^4-3^2017. 3^2+3^2017 .1`
`=3^2017 .(3^4-3^2+1)`
`=3^2017 .73\vdots73`
Vậy `3^2021-3^2019+3^2017\vdots73`
`b,` `2^(n+2)+2^(n+1)+2^n`
`=2^n .2^2+2^n .2+2^n .1`
`=2^n .(2^2+2+1)`
`=2^n .7\vdots7`
Vậy `2^(n+2)+2^(n+1)+2^n\vdots7`
`c,` `6^6-3^5`
`=(2.3)^6-3^5`
`=2^6 .3^6-3^5`
`=2^6 .3.3^5-3^5 .1`
`=3^5 .(2^6 .3-1)`
`=3^5 .191\vdots191`
Vậy `6^6-3^5\vdots191`
