Réponse :

Exercice 1

A =  5/7 + 2 = 5/7 + 14/7 = 19/7

B = 20/12 - 1/3 = 20/12 - 4/12 = 16/12 = 4/3

C = 7/4 + 2/9 = 63/36 + 8/36 = 71/36

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Exercice 2

A = -4/3 * 3/5 = -4/5

B = -1/2 * 2/3 * -3/4 * -6/5 = -3/10

C = 13/7 * -14 * 5/26 = -5/1 = -5

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Exercice 3

A = -25/12 : 9/4 = -25/12 * 4/9 = -25/27

B = (-15/8)/3 = -15/(8*3) = -15/24 = -5/8

C = (5/9)/(-2/3) = 5/9 * -3/2 = -5/6