Bài 2:
`c1:5/9 + ( (-4)/9 + 2 ) = (5/9 + (-4)/9 ) + 2 = 1/9 + 2 = 19/9 `
`c2:5/11 + ( (-6)/11 + 1 ) = ( 5/11+ (-6)/11 ) +1 = -1/11 + 1 = 10/11`
`c3:(-6)/13 + (1 + (-7)/13) = (-6/13+ (-7)/13 ) + 1 = -1 + 1 = 0`
`c4:(-7)/31+(99+ (-7)/31)=( (-7)/31+(-7)/31 )+99= -14/31+99= 3055/31 `
`d1: 1/2 + 1/3 + 1/4 + 1/6`
`= (6+4+3)/12 + 1/6`
`= 13/12 + 2/12`
`= 15/12 = 5/4`
`d2: 2/3 + (-3)/4 + 5/8 + (-1)/2 `
`= (16+(-18) +15)/24 +(-1)/2 `
`= 13/24 + (-1)/2 `
`= 13/24 + (-12)/24 `
`= 1/24`
`d3: 5/17 + (-9)/15 + (-2)/17 + 2/(-5)`
`= (5/17+ (-2)/17) + ((-9)/15 + (-6)/15) `
`= 3/17 + -15/15 `
`= 3/17 + (-1) `
`= -14/17`
`d4: -1/2 + 7/21 + (-2)/6 + (-6)/30 `
`= -1/2 + 1/3 + (-1/3) + (-1/5) `
`= (1/3 + (-1)/3 ) + ( (-5)/10 + (-2)/10 ) `
`= 0 + (-7)/10`
`= -7/10`
