Bài `1:`
`8=2^3`
`20=2^2.5`
`=>ƯCLN=2^2=4`
__________________________________________
Bài `2:`
`a)45=3^2.5`
`117=3^2.13`
`=>ƯCLN=3^2=9`
~~~~~~~~~~
`b)390=2.3.5.13`
`208=2^4 .13`
`=>ƯCLN=2.13=26`
~~~~~~~~~~
`c)24=2^3 .3`
`36=2^2.3^2`
`60=2^2.3.5`
`=>ƯCLN=2^2.3=4.3=12`
~~~~~~~~~~
`d)16=2^4`
`64=2^6`
`400=2^4.5^2`
`=>ƯCLN=2^4=16`