Bài 1: Thực hiện phép tính

a) -5/4 + 9/4 => 4/4 => 1
b) 5/14 - 9/14 => -4/14 => -2/7
c) -5/7 - 10/7 => -15/7
d) 10/14 + 5/14 => 15/14 => 1 1/14
e) 12/7 - 3/19 + 16/19 => 12/7 + 13/19 => (228 + 91)/133 => 319/133
f) -5/7 + 2/11 - 5/7 + 9/11 + 1/7 => (-5 - 5 + 1)/7 + (2 + 9)/11 => -9/7 + 11/11 => -9/7 + 1 => -2/7
g) 3/5 - 9/10 - 7/6 => (18/30 - 27/30 - 35/30) => -44/30 => -22/15
h) 21/36 - 18/7 => 7/12 - 18/7 => (49 - 216)/84 => -167/84
i) 3/7 - 1/21 => (9 - 1)/21 => 8/21

Bài 2: Thực hiện phép tính

a) 25% × -9.8 + 3.75 => -2.45 + 3.75 => 1.3
b) 4.5 × (0.5 - 0.36 : 6 + (-0.8))
=> 4.5 × (0.5 - 0.06 - 0.8)
=> 4.5 × (-0.36)
=> -1.62

Bài 3: Tính nhanh các tổng

a) -3/4 + 2/4 - 1/4 + 3/5 + 5/7
=> (-2/4) + 3/5 + 5/7 => -1/2 + 3/5 + 5/7
=> (-35 + 42 + 50)/70 => 57/70
b) (13/18 + 1/71) - (13/18 + 70/71)
=> (13/18 - 13/18) + (1/71 - 70/71) => -69/71
c) 8/9 - 7/13 + 8/9 - 6/13 + 2×8/9
=> (8/9 + 8/9 + 16/9) + (-7/13 - 6/13)
=> 32/9 - 13/13 => 32/9 - 1 => 23/9

Bài 4: Tìm x

a) 2/5 + x = 2/7 => x = 2/7 - 2/5 => (10 - 14)/35 => -4/35
b) -2/3 + 2x = 4/3 => 2x = 4/3 + 2/3 => 6/3 => 2 => x = 1
c) 5/7 - 4x = 51/7 => -4x = 46/7 => x = -46/28 => -23/14
d) 7/12 + x/15 = 1/20 => x/15 = 1/20 - 7/12 => (3 - 35)/60 => -32/60 => -8/15 => x = -8
e) -2/5 + 4/5×x = 3/5 => 4/5×x = 5/5 => x = 1 ÷ (4/5) => 5/4