Bonjour !
A = 1/4 - 1/4 × 2/3
= 1/4 - 1×2/4×3
= 1/4 - 1×2/2×2×3
= 1/4 - 1/6
= 1×3/4×3 - 1×2/6×2
= 3/12 - 2/12
= 1/12
B = 2/3 + 1/2 / 17/9 - 1/3
= (2/3 + 1/2) ÷ (17/9 - 1/3)
= (2×2/3×2 + 1×3/2×3) ÷ (17/9 - 1×3/3×3)
= (4/6 + 3/6) ÷ (17/9 - 3/9)
= 7/6 ÷ 14/9
= 7/6 × 9/14
= 7×9/6×14
= 7×3×3/2×3×2×7
= 3/4
