a/ $\dfrac{5}{10}=\dfrac{5:5}{10:5}=\dfrac{1}{2}$

$\dfrac{25}{75}=\dfrac{25:25}{75:25}=\dfrac{1}{3}$

Quy đồng, mẫu số chung $6$

$\dfrac{1}{2}=\dfrac{1×3}{2×3}=\dfrac{3}{6}$

$\dfrac{1}{3}=\dfrac{1×2}{3×2}=\dfrac{2}{6}$

b/ $\dfrac{42}{56}=\dfrac{42:14}{56:14}=\dfrac{3}{4}$

$\dfrac{18}{48}=\dfrac{18:6}{48:6}=\dfrac{3}{8}$

Quy đồng, mẫu số chung $8$

$\dfrac{3}{4}=\dfrac{3×2}{4×2}=\dfrac{6}{8}$

$\dfrac{3}{8}=\dfrac{3}{8}$

c/ $\dfrac{27}{81}=\dfrac{27:27}{81:27}=\dfrac{1}{3}$

$\dfrac{57}{76}=\dfrac{57:19}{76:19}=\dfrac{3}{4}$

$\dfrac{35}{84}=\dfrac{35:7}{84:7}=\dfrac{5}{12}$

Quy đồng, mẫu số chung $12$

$\dfrac{1}{3}=\dfrac{1×4}{3×4}=\dfrac{4}{12}$

$\dfrac{3}{4}=\dfrac{3×3}{4×3}=\dfrac{9}{12}$

$\dfrac{5}{12}=\dfrac{5}{12}$

Đáp án:+Giải thích các bước giải:

`a)`
Ta có:
`5/10=(5:5)/(10:5)=1/2`
`25/75=(25:25)/(75:25)=1/3`
Quy đồng:MSC:`6`
`1/2=(1xx3)/(2xx3)=3/6`
`1/3=(1xx2)/(3xx2)=2/6`
`b)`
Tan có:
`42/56=(42:14)/(56:14)=3/4`
`18/48=(18:6)/(48:6)=3/8`
Quy đồng:MSC:`8`
`3/4=(3xx2)/(4xx2)=6/8`
`3/8=3/8`
`c)`
Ta có:
`27/81=(27:27)/(81:27)=1/3`
`57/76=(57:19)/(76:19)=3/4`
`35/84=(35:7)/(84:7)=5/12`
Quy đồng:MSC:`12`
`1/3=(1xx4)/(3xx4)=4/12`
`3/4=(3xx3)/(4xx3)=9/12`
`5/12=5/12`