Bài 1:
Ta có:
$8=2^3$
$20=2^2.5$
$\Rightarrow ƯCLN(8,20)=2^2=4$
Bài 2:
a.
$45=3^2.5$
$117=3^2.113$
$\Rightarrow ƯCLN(45,117)=3^2=9$
c.
$24=2^3.3$
$36=2^2.3^2$
$60=2^2.3.5$
$\Rightarrow ƯCLN(24,36,60)=2^2.3=12$
b.
$390=2.3.5.13$
$208=2^4.13$
$\Rightarrow ƯCLN(390,208)=2.13=26$
d.
$16=2^4$
$64=2^6$
$400=2^4.5^2$
$\Rightarrow ƯCLN(16,64,400)=2^4=16$