bonjour pouvez vous m'aider svp ex 11 merci

bonjour

a = 5/4 + 7/8
a = 10/8 + 7/8 = 17/8

b = - 2/5 + 7/15
b = - 6/15 + 7/15
b = 1/15

c = 8/18  - 7/6
c = 8/18 - 21/18
c = - 3/18 = - 1/6

d = -5/32 - 3/8
d = - 5/32 - 12/32
d = - 17/32