9)
`5/4. (-5)/7 +(-5)/7 : 4/9`
`=5/4. (-5)/7 +(-5)/7 .9/4`
`=(-5)/7 . (5/4+9/4)`
`=(-5)/7 . 14/4`
`=(-5)/2`
10)
`(-3/5):9/4-(-3/5):5/4`
`=(-3/5). 4/9-(-3/5). 4/5`
`=(-3/5)(4/9-4/5)`
`=(-3/5)(-16/45)`
`=16/75`
11)
`4/7 . 5/9 + +2/7 . 4/9 `
`=4/9 . 5/7 + 2/7 . 4/9 `
`= 4/9 (5/7+2/7)`
`= 4/9 `
12)
`(-5)/2. 7/8 +(-5)/8 . 9/2`
`=(-5)/2 . 7/8 +(-5)/2 . 9/8`
`=(-5/2)(7/8+9/8)`
`=(-5/2).2`
`=-5`
