Đáp án:

a, -5/ 8 + -7 / 20

 msc : 40 

 ta quy Đong : -5/8 =-5 x 5 /8 x 5 = -25 / 40 

                        - 7/ 20 = -7 x 2 / 20 x 2 = -14 / 40

 => -25/40 + -14 /40 = -39 /40

 vay - 5/8 + -7 / 20 = -39 / 40 

b, 7/21 + 9/ - 36 

msc ; 756 

 7/ 12 = 7 x 36 / 21 x 36 = 252 / 756 

 9 / -36 = 9 x 12 / -36 x 12 = 108 / - 756 

=> 252 / 756  + 108 / - 756 =4/ 21

vay 7/ 21 + 9/ - 36 = 4/ 21

c, 8/ 40 + - 36 / 45 

msc : 1800 

 8/ 40 = 8 x 45 / 40 x 45 = 360 / 1800

 -36 / 45 = -36 x 40/ 45 / 40 = - 1440 / 1800

  => 360 / 1800 + - 1440 / 1800 = - 3/ 5

vay 8/ 40  + - 36 / 45 =-3/5 

d, -8/18 + -15/27 

msc : 486 

-8/18 = -8 x 27 / 18 x 27 = - 216 / 486 

- 15 / 27 = - 15 x 18 / 27 x 18 = - 270 / 486 

=> -216 / 486 + - 270 / 486 = - 1

vay -8/18 + - 15 / 27 = - 1

bÀI 2 : 

 a, 2-(-4/5)

= 14/ 5

b, (-2) + -5/8 

= -21/8

c, 3/-4 -(-3)

 = 9/4 

 d, 6- (-3/20)

 =123/20