Bonjour
a. Ce nombre est 42
b. 1/6 + 1/7 = (1 x 7) / (6 x 7) + (1 x 6) / (7 x 6) = 7/42 + 6/42 = 13/42
c. 5/6 - 9/7 = (5 x 7) / (6 x 7) - (9 x 6) / (7 x 6) = 35/42 - 54/42 = - 19/42
d. (1/6 + 1/7) - (5/6 - 9/7)
= (7/42 + 6/42) - (35/42 - 54/42)
= 13/42 - (- 19/42)
= 13/42 + 19/42
= 32/42
= (2 x 16) / (2 x 21)
= 16/21.
