Răspuns:
r=ratie
Explicație pas cu pas:
[tex]a_{7}= a_{4} + (7-4)r\\a_{7}=a_{4} +3r\\19= 10+3r\\3r=19-10\\3r=9\\r=3[/tex]
Acum aflam [tex]a_{1}[/tex]:
[tex]a_{7}=a_{1} + (7-1)r\\a_{7}= a_{1} +6r\\19= a_{1} +6*3\\19=a_{1} + 18\\a_{1}= 1[/tex]
Deci am obtinut : r=3 si [tex]a_{1}[/tex]=1
