bonsoir

a )  3/4 = x /12

     4 x = 36

      x = 9

b) x/16 = 3/12

   12 x =  48

     x =  4

c) 8/x = 24/15

    24 x =  120

    x = 5

d)  2.1/4 = x /12

     4 x =  25.2

      x = 6.3

d'après le codage de la figure, ARS est rectangle en R

AS² = AR² + RS²

AS² = 2.1 ² + 2.8²

AS² =  12.25

AS = √12.25 =  3.5 cm

AR/AB = AS/AC = RS/BC

2.1/6 = 2.8/BC

BC = ( 6 * 2.8) : 2.1 =  8 cm

3.5/AC = 2.8/8

AC = ( 3.5 * 8) : 2.8 =  10 cm