Bonjour ;

Exercice n° 5 .

a.

3/7 + 2/7 = (3 + 2)/7 = 5/7 .

3/7 - 2/7 = (3 - 2)/7 = 1/7 .

3/7 x 2/7 = (3 x 2)/(7 x 7) = 6/49 .

3/7 ÷ 2/7 = 3/7 x 7/2 = (3 x 7)/(2 x 7) = 3/2 .

b.

On a : 1/2 = (1 x 4)/(2 x 4) =4/8 .

1/2 + 3/8 = 4/8 + 3/8 = (4 + 3)/8 = 7/8 .

1/2 - 3/8 = 4/8 - 3/8 = (4 - 3)/8 = 1/8 .

1/2 x 3/8 = (1 x 3)/(2 x 8) = 3/16 .

1/2 ÷ 3/8 = 4/8 ÷ 3/8 = 4/8 x 8/3 = (4 x 8)/(3 x 8) = 4/3 .

c.

On a : 5/9 = (5 x 2)/(9 x 2) = 10/18 ,

et : 1/6 = (1 x 3)/(6 x 3) = 3/18 .

5/9 + 1/6 = 10/18 + 3/18 = (10 + 3)/18 = 13/18 .

5/9 - 1/6 = 10/18 - 3/18 = (10 - 3)/18 = 7/18 .

5/9 x 1/6 = (5 x 1)/(9 x 6) = 5/54 .

5/9 ÷ 1/6 = 10/18 ÷ 3/18 = 10/18 x 18/3 = (10 x 18)/(3 x 18) = 10/3 .

d.

On a : 3 = 3/1 = (3 x 4)/(1 x 4) = 12/4 .

3 + 1/4 = 12/4 + 1/4 = (12 + 1)/4 = 13/4 .

3 - 14 = 12/4 - 1/4 = (12 - 1)/4 = 11/4 .

3 x 1/4 = 3/1 x 1/4 = (3 x 1)/(1 x 4) = 3/4 .

3 ÷ 1/4 = 3/1 ÷ 1/4 = 3/1 x 4/1 = (3 x 4)/(1 x 1) = 12/1 = 12 .