`(12/32+ -5/20 - 10/34)`
`= 3/8 - 1/4 - 5/17`
`= 51/136 - 34/136 - 40/136`
`= (51 - 34 - 40)/136`
`=-23/136`
`4 1/2 : (2/5 - 3 3/4). (-1/2)^2`
`= 9/2 : (2/5 - 15/4). 1/4`
`= 9/2 : (8/20 - 75/20). 1/4`
`= 9/2 : -67/20 . 1/4`
`= 9/2. -20/67. 1/4`
`= -90/67. 1 /4`
`= -45/134`