Réponses

2014-01-31T19:25:57+01:00
A = 3/ 7 + 26/21= 9/21 + 26/21 = 35/21
B =  1/12 +1/6 = 1/12 +2/12 = 3/12
C = 21/12 - 2/3 = 21/12 - 8/12 = 13/12
D = 57/45 -9/15 = 57/45 - 27/45 = 30/45

2/
 1/3 +4/3 = 5/3
1/3+11/6 = 2/6 + 11/6 = 13/6
1/3+7/12 = 4/12 +7/12 = 11/12

5/6 + 4/3 = 5/6 +8/6 =13/6
5/6+11/6 = 16/6
5/6+7/12 = 10/12+7/12 = 17/12

2014-01-31T19:33:45+01:00
25)

A = 3/7 + 26/21
A = (3*3)/(3*7) + 26/21
A = 9/21 + 26/21
A = 35/21
A = (7*5)/(7*3)
A = 5/3

B = 1/12 + 1/6
B = 1/12 + (2*1)/(2*6)
B = 1/12 + 2/12
B = 3/12
B = (3*1)/(3*4)
B = ¼

C = 21/18 - ⅔
C = 21/18 - (6*2)/(6*3)
C = 21/18 - 12/18
C = 9/18
C = (9*1)/(9*2)
C = ½

D = 57/45 - 9/15
D = 57/45 - (3*9)/(3*15)
D = 57/45 - 27/45
D = 30/45
D = (15*2)/(15*3)
D = ⅔

26)

· ⅓ + 4/3 = 5/3
· ⅓ + 11/6 = (2*1)/(2*3) + 11/6 = 2/6 + 11/6 = 13/6
· ⅓ + 7/12 = (4*1)/(4*3) + 7/12 = 4/12 + 7/12 = 11/12

· 5/6 + 4/3 = 5/6 + (2*4)/(2*3) = 5/6 + 8/6 = 13/6
· 5/6 + 11/6 = 16/6 = (2*8)/(2*3) = 8/3
. 5/6 + 7/12 = (2*5)/(2*6) + 7/12 = 10/12 + 7/12 = 17/12

Voilà ;) C'est très,très,très ... (trop) détaillé , mais j'espère que ça t'aideras à comprendre ;)