Đáp án:

 a) A = 2/3 - (-1)/4 + 3/5 - 7/45 - (-5)/9 + 1/12 + 1/90

A = 2/3 + 1/4 + 3/5 - 7/45 + 5/9 + 1/12 + 1/90

A = (2/3 + 1/4 + 1/12) + (3/5 - 7/45 + 5/9) + 1/90

A = 1 + 1 + 1/90

A = 2 + 1/90

A = 181/90

b) (7 - 4/3 + 1/3) - (6 + 5/4 - 4/3) - (5 - 7/4 + 5/3)

= 7 - 4/3 + 1/3 - 6 - 5/4 + 4/3 - 5 + 7/4 - 5/3

= (7 - 6 - 5) + (-4/3 + 1/3 + 4/3 - 5/3) + (-5/4 + 7/4)

= (-4) + (-4/3) + 1/2

= -29/6

c) C = 3/4 - [-3/5 - (1/12 + 2/9) - (-2/9 - 5/3)]

C = 3/4 - [-3/5 - 1/12 - 2/9 + 2/9 + 5/3]

C = 3/4 - [-3/5 + (-1/12 + 5/3) + (-2/9 + 2/9)]

C = 3/4 - [-3/5 + 19/12 + 0]

C = 3/4 - 59/60

C = -7/30

d) D = 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/19.20

D = 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + ... + 1/19 - 1/20

D = 1/2 + (-1/3 + 1/3) + (-1/4 + 1/4) + (-1/5 + 1/5) + ... + (-1/19 + 1/19) - 1/20

D = 1/2 + (-1/20)

D = 9/20