1,21
`a) 3/4 + \frac{-1}{8}`
`= 6/8 + \frac{-1}{8}`
`= 5/8`
---------------------------------------
`b) -5/12 + \frac{-7}{24}`
`= -10/24+ \frac{-7}{24}`
`= -17/24`
--------------------------------------
`c) 1 + \frac{-7}{8}`
`= 8/8 + \frac{-7}{8}`
`= 1/8`
----------------------------------------
`d) 13/30 - 1/3`
`= 13/30 - 10/30`
`= 1/10`
--------------------------------------------
`e) 4/21 - \frac{-5}{28}`
`= 16/84 - \frac{-15}{84}`
`= 31/84`
------------------------------------------
`g) - 1 1/3 - 2 5/6`
`= -4/3 - 17/6`
`=-25/6`
==================================
bài 1.22)
`a) 1/3 - ( 1/2 + 1/8)`
`= 1/3 - 1/2 - 1/8`
`= 8/24 - 12/24 - 3/24`
`= -7/24`
-------------------------------
`b) 1/21 - ( 1/7 - 1/3)`
`= 1/21 - 1/7 + 1/3`
`= 1/21 - 3/21 + 7/21`
`= 5/24`
-------------------------------
`c) 1/2 - 1/4 + 1/13 - 1/8`
`= 52/104 -26/104 +8/104 - 13/104`
`= 21/104`
==========================================
1,23
`a) x - 1/10 = 1/15`
`x = 1/15 + 1/10`
`x =1/6`
---------------------------------
`b) -4/21 - x = -3/7`
`x = -4/21 - (-3/7)`
`x = 5/21`
==================================
`1,24`
`a) x + 1/2 = 3/4 - (-1/2)`
`x + 1/2 = 5/4`
`x = 5/4 - 1/2`
`x = 3/4`
-----------------------------------
`b) 4/7 - x = 1/3 - (-2/5)`
`4/7 - x = 11/15`
`x = 4/7 - 11/15`
`x =-17/105`
==============================
1.25
`A = 1/3 - 3/5 + 5/7 - 7/9 + 9/11 + 7/9 - 5/7 + 3/5 - 1/3`
`= ( 1/3 - 1/3) - ( 3/5 - 3/5) - ( 5/7 - 5/7) - ( 7/9 - 7/9) + 9/11`
`= 0 - 0 - 0 - 0 + 9/11`
`= 9/11`