`4/5=(4xx4)/(5xx4)=16/20`
`3/4=(3xx5)/(4xx5)=15/20`
Do `16>15` Vậy : `16/20>15/20`
`4/5>3/4`
_____________________________
`5/15=(5:5)/(15:5)=1/3`
`7/21=(7:7)/(21:7)=1/3`
`1/3=1/3`
Vậy : `5/15=7/21`
___________________
`3/7=(3xx11)/(7xx11)=33/77`
`5/11=(5xx7)/(11xx7)=35/77`
`33<35` Vậy `33/77<35/77`
`3/7<5/11`