Bonjour,

50)

a. 7/6 - 9/8 = (7 x 4) / (6 x 4) - (9 x 3) / (8 x 3) = 28/24 - 27/24 = 1/24

b. - 7/12 + 7/9 = (- 7 x 3) / (12 x 3) + (7 x 4) / (9 x 4) = - 21/36 + 28/36 = - 3/36 = (3 x - 1) / (3 x 12) = - 1/12.

51) 1-

9 ; 18 ; 27 ; 36 ; 45

12 ; 24 ; 36 ; 48 ; 60 ; 72

2-

a. - 7/9 + 5/12 = (- 7 x 4) / (9 x 4) + (5 x 3) / (12 x 3) = - 28/36 + 15/13 = - 13/36

b. - 7/12 + 7/9 = - 21/36 + 28/36 = 7/36.