a) `[( 1/9 : 8/27) : (-1)/3 ] : 81/128`
`= (1/9 . 27/8) . (-3) . 128/81`
`= 3/8 . (-3) . 128/81`
`=-9/8 . 128/81`
`=-16/9`
b) `-7/15 . (-5)/8 . (15/-7) . (-32)`
`= [ -7/15 . (15/-7)] . [ (-5/8 . (-32)]`
`= 1 . 20`
`= 20`
c) `8/9 - 1/72 - 1/56 -...- 1/6 - 1/2`
`= -1/2 - 1/6 -... -1/56 - 1/72 + 8/9`
`= - (1/2 + 1/6 + ...+ 1/56 + 1/72) + 8/9`
`= - (1/1.2 + 1/2.3 +...+ 1/7.8 + 1/8.9) + 8/9`
`= - ( 1/1 - 1/2 + 1/2 - 1/3+...+ 1/7- 1/8 + 1/8 - 1/9 ) + 8/9`
`= - ( 1 - 1/9) + 8/9`
`= - 8/9 + 8/9`
`=0`