Réponse :

1.

[tex]U_n =3n+6\\U_{n+1}= 3(n+1)+6\\U_{n+1}= 3n+3+6\\U_{n+1}=U_n+3[/tex]

2.

[tex]U_n= n^{2} \\U_{n+1}= (n+1)^{2} \\U_{n+1}= n^{2}+2n+1\\U_{n+1}= U_n+2n+1[/tex]

3.

[tex]U_n=2n^2\\U_{n+1}=2(n+1)^2\\U_{n+1}=2(n^2+2n+1)\\U_{n+1}=2n^2+4n+2\\U_{n+1}=U_n+4n+2[/tex]