`a) (-3)/5 + 7/21 + (-4)/5 + 7/5`
`= ( (-3)/5 + (-4)/5 + 7/5) + 7/21`
`= 0 + 1/3`
`=1/3`
``
`b) (-3)/17 + (2/3 + 3/17)`
`= (-3)/17 + 2/3 + 3/17`
`= ((-3)/17 + 3/17) + 2/3`
`= 0 + 2/3`
`= 2/3`
``
`c) (-5)/21 + ((-16)/21 + 1)`
`= (-5)/21 - 16/21 + 1`
`= ((-5)/21 - 16/21) + 1`
`= (-1) + 1`
`= 0`
``
`d) 5/7 + 9/23 + (-12)/7 + 14/23`
`= (5/7 + (-12)/7) + ( 9/23 + 14/23)`
`= -1 + 1`
`= 0`
``
`e) 3/17 + (-5)/13 + (-18)/35 + 14/17 + 17/(-35) + (-8)/13`
`= ( 3/17 + 14/17) + ((-5)/13 + (-8)/13) + ((-18)/35 + (-17)/35)`
`= 1 + (-1) + (-1)`
`= (-1)`
