Réponse :

Explications étape par étape :

a.

a+b= 3/4 + (-1/2) = 3/4+ (-2/4) = 1/4

1/(a+b) = 1/(1/4) = 1x (4/1) = 1x(4) = 4

b. 1/a = 4/3  et 1/b = -2/1 = -2

1/a + 1/b = 4/3 + (-2) = 4/3 + (-6/3) = -2/3