Bonjour,
A= 2/3*7/4-5/8:3/7
A = 14/12 - 5/8 x 7/3
A = 14/12 - 35/24
A = 14 x 2 / 12 x 2 - 35/24
A = 28/24 - 35/24
A = - 7/24 - OK
B=(4/5)²:4/-15
B = 4²/5² x - 15/4
B = 16/25 x - 15/4
B = - 240/100
B = 20 x - 12 / 20 x 5
B = - 12/5
C= 1/1-(1-1/6)
C = 1/1 - (1 x 6 / 1 x 6 - 1/6)
C = 1 x 6 / 1 x 6 - (6/6 - 1/6)
C = 6/6 - 5/6
C = 1/6