Gọi `d` là `ƯCLN(2n+1,3n+1)`

`=>2n+1 vdots d ; 3n+1 vdots d`

`=>3(2n+1) vdots d ; 2(3n+1) vdots d`

`=>6n+3 vdots d ; 6n+2 vdots d`

`=>(6n+3-6n-2) vdots d`

`=>1 vdots d`

`=>d=1`

Vậy `2n+1` và `3n+1` là `2` số nguyên tô cùng nhau

`1+9=10`

`15+20`

`=35`

`1+1=2`

`2+2=3`

`3+3=6`

`1-1=0`

`2+9=11`

`2+8=10`

`9-9=0`

`15+4+9`

`=19+9`

`=28`

`1+1=2`

1

+

1

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2

Cách `1:`

`1+1+1+1`

`=2+1+1`

`=3+1`

`=4`

Cách `2:`

`1+1+1+1`

`=1xx4`

`=4`

`1:1+1xx1:2`

`=1+1:2`

`=1+1/2`

`=3/2`

`789.15+789.5+789.80`

`=789.(15+5+80)`

`=789.100`

`=78900`