bonjour
a = 2/3 ; b = 1/4 ; c = 2/5 ; d = -1/2
A = (2/3)(1/4) + (2/5)(-1/2)
= (2 x 1)/(3 x 4) - (2 x 1)/(5 x 2)
= (1 x 1)/ (3 x 2) - (1 x 1) / (5 x 1)
= 1/6 - 1/5
= 5/30 - 6/30 = -1/30
B
a + d = 2/3 - 1/2 = 4/6 - 3/6 = 1/6
b + c = 1/4 + 2/5 = 5/20 + 8/20 = 13/20
B = (1/6) / (13/20) = (1/6) x (20/13) = 20 / (6 x 13) = 10/(3 x 13) = 10/39
C = a - b = 2/3 - 1/4 = 8/12 - 3/12 = 5/12
