exercice 22
a. 42
b. 1/6 + 1/7 = 7/42 + 6/42 = 13/42 (on met au dénominateur commun)
c. 5/6 - 9/7 = 35/42 - 54/42 = -19/42 (on met au dénominateur commun)
d. (1/6 + 1/7) - (5/6 - 9/7) = 13/42 - (-19/42) = 13/42 + 19/42 = 16/21
exercice 23
a. 3/9 + 5/2 = 6/18 + 45/18 = 17/6
b. 6/7 - 2/3 = 18/21 - 14/21 = 4/21
c. 7/4 - 9/5 = 35/20 - 36/20 = -1/20
d. 3/8 + 1/7 = 21/56 + 8/56 = 29/56
e. -7/3 + -1/4 = -28/12 - 3/12 = -31/12
f. 2/11 - 3/10 = 20/110 - 33/110 = -13/110
