Bonjour Willy

Ce ne sont pas des équations. Ce sont simplement des expressions à calculer
non ?

a = 3/7 + 4/21 - 5/2
= 18/42 + 8/42 - 105/42
= (18+8-105)/42
= -79/42


b = 2/3 - 4/3 × 5 - 2
= 2/3 - 20/3 - 2
= 2/3 - 20/3 - 6/3
= (2-20-6)/3
= -24/3
= -(3×8)/3
= -8


c = 5 + (1 + 1/8) × 3/4
= 5 + (8/8 + 1/8)×3/4
= 5 + 9/8 × 3/4
= 5 + (9×3)/(8×4)
= 5 + 27/32
= 160/32 + 27/32
= 187/32


d = 3/5 - 1/5 × (5/2 + 2)
= 3/5 - 1/5 × (5/2 + 4/2)
= 3/5 - 1/5 × 9/2
= 3/5 - (1×9)/(5×2)
= 3/5 - 9/10
= 6/10 - 9/10
= -3/10


e = 3/7 + 4/21 - 5/2
C'est la même expression que la a)

A toi de tout reprendre, bon courage ...