Bài 15
`4/7 × 5/6 + 4/7 × 1/6`
`= 4/7 × ( 5/6 + 1/6 )`
`= 4/7 × 1`
`= 4/7`
`3/5 × 7/9 - 3/5 × 2/9`
`= 3/5 × ( 7/9 - 2/9 )`
`= 3/5 × 5/9`
`= 1/3`
`5/9 × 1/4 + 4/9 × 3/12`
`= 5/9 × 3/12 + 4/9 × 3/12`
`= 3/12 × ( 5/9 + 4/9 )`
`= 3/12 × 1`
`= 3/12`
`7/9 × 8/5 - 7/9 × 3/5`
`= 7/9 × ( 8/5 - 3/5 )`
`= 7/9 × 1`
`= 7/9`
`2/5 × 3/4 + 3/4 × 3/5`
`= 3/4 × ( 2/5 + 3/5 )`
`= 3/4 × 1`
`= 3/4`
`2006/2005 × 3/4 - 3/4 × 1/2005`
`= 3/4 × ( 2006/2005 - 1/2005 )`
`= 3/4 × 1`
`= 3/4`
Bài 16
`1/2 × 12/13 + 1/3 × 12/13 + 1/4 × 12/13`
`= ( 1/2 + 1/3 + 1/4 ) × 12/13`
`= 13/13 × 12/13`
`= 12/13`
`( 1 - 1/2 ) × ( 1 - 1/3 ) × ( 1 - 1/4 ) × ( 1 - 1/5 )`
`= 1/2 × 2/3 × 3/4 × 4/5`
`= {1 × 2 × 3 × 4}/{2 × 3 × 4 × 5}`
`= 1/5`
Bài 17
`a ) 15 × 2121/4343 + 15 × 222222/434343`
`= 15 × 21/43 + 15 × 22/43`
`= 15 × ( 21/43 + 22/43 )`
`= 15 × 1`
`= 15`
`#dtkc`
