Fraction puissance 3ème Ex 73, pouvez vous m'expliquer?

Exercice 73 :

a]
1/3 + 2/5 - 3
= 5/15 + 6/15 - 45/15
= 11/15 - 45/15
= - 34/15

b]
6 - 4(1/4 - 1)²
= 6 - 4(1/4 - 4/4)²
= 6 - 4(- 3/4)²
= 6 - 4 x 9/16
= 6 - 36/16
= 6 - 9/4
= 24/4 - 9/4
= 15/4

c]
- 5/14 + 3 x 1/42
- 15/42 + 3/42
= - 12/42
= - 6/21

d]
(1 - 1/3)/(1/4 - 2)
= (3/3 - 1/3)/(1/4 - 8/4)
= (2/3)/(- 7/4)
= - 2/3 x 4/7
= - 8/21