Bonjour
a = 1/2 ; b = - 2/3 ; c = 5/4
A = a + b + c
A = 1/2 - 2/3 + 5/4
A = (1 x 12) / (1 x 12) - (2 x 4) / (3 x 4) + (5 x 3) / (4 x 3)
A = 12/12 - 8/12 + 15/12
A = 19/12
B = a - b - c
B = 1/2 - (- 2/3) - 5/4
B = 12/12 + 8/12 - 15/12
B = 5/12
C = a - (b - c)
C = 1/2 - (- 2/3 - 5/4)
C = 12/12 - (- 8/12 - 15/12)
C = 12/12 - (- 22/12)
C = 12/12 + 22/12
C = 10/12
C = (2 x 5) / (2 x 6)
C = 5/6.