Réponse:
a = 3/4 + 2/6
= 3/4 + 1/3
= 9/12 + 4/12
=13/12
b = 2/15 + 3/20
= 2x20/15x20 + 3x15/20x15
=40/300 + 45/300
=85/300
=17/60
c= 5/12-9/8
=5x8/12x8 - 9x12/8x12
=40/96 - 108/96
=40-108/96
=-68/96
=-17/24
