`5/9 xx 3 = 15/9`
`6 : 7/5 = 6 xx 5/7 = 30/7`
`( 36 xx 22 xx 51 )/( 11 xx 17 xx 72 )`
`= ( 2 xx 2 xx 3 xx 3 xx 11 xx 2 xx 3 xx 17 )/( 11 xx 17 xx 2 xx 3 xx 2 xx 2 xx 3 )`
`= 3`
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Ta có `: 2012/2013 = ( 2013 - 1 )/2013 = 1 - 1/2013`
`1998/1999 = ( 1999 - 1 )/1999 = 1 - 1/1999`
Vì `1/1999 > 1/2013` nên `1998/1999 < 2012/2013`
Vậy `; 1998/1999 < 2012/2013 .`