Respuesta :

Answer:

0.06

Step-by-step explanation:

let 0.09 be a1

and 0.15 be a2

and 0.21 be a3

common difference = a2-a1 or a3-a2

a2-a1= 0.15-0.09 = 0.06

a3-a2= 0.06

hence the common difference in the ap in 0.06