`1,`
`a, {-1}/21 + {-2}/7`
`= {-1}/21 + {-6}/21`
`= {-7}/21`
`= {-1}/3`
`b, 3/7 + {-5}/2 + {-3}/5`
`= -29/14 - 3/5`
`= -187/70`
`c, (-2/3 + 3/7) . 4/5 + (-1/3 + 4/7) . 4/5`
`= 4/5 . (-2/3 + 3/7 - 1/3 + 4/7)`
`= 4/5 . [(-2/3 - 1/3) + (3/7 + 4/7)]`
`= 4/5 . [-1 + 1]`
`= 4/5 . 0`
`= 0`
__________________
`2,`
`a, x + 1/2 = 3/11`
`x = 3/11 - 1/2`
`x = -5/22`
$\text{Vậy}$ `x = -5/22`
`b, x - 2/3 = 1/5 . {-5}/4`
`x - 2/3 = -1/4`
`x = -1/4 + 2/3`
`x = 5/12`
$\text{Vậy}$ `x = 5/12`
