Bonjour,
A=
1/2 -2/3 +5/4
= 12/24 -16/24 +30/24
= 26/24
= 13/12
B=
1/2 +2/3 -5/4
= 12/24 + 16/24 -30/24
= 8/24
= 4/12
= 2/6
= 1/3
C=
1/2-( -2/3 -5/4)
= 12/24-( -16/24 -30/24)
= 12/24 + 46/24
= 58/24
= 29/12