a) `(-5/12 +6/11)+(7/17+5/11+5/12)`
`= -5/12+6/11+7/17+5/11+5/12`
`=(-5/12+5/12)+(6/11+5/11)+7/17`
`=0+1+7/17`
`=27/17`
b) `-2/11+6/7+1/2+(-9/11)+1/7`
`=[-2/11+(-9/11)]+(6/7+1/7)+1/2`
`=-1+1+1/2`
`=1/2`
c) `[9/16 + (-8/27)]+[1+7/16+(-19/27)]`
`=9/16+(-8/27)+1+7/16+(-19/27)`
`=(9/16+7/16)+[(-8/27)+(-19/27)]+1`
`=1+(-1)+1`
`=1`
d) `8/19+4/21+2/5+17/21+(-27/19)`
`=[(8/19+(-27/19)]+(4/21+17/21)+2/5`
`=-1+1+2/5`
`=2/5`