Question

Se considera progresia aritmetica (an) in care a3=5 si a6=11. Sa se calculeze a9.

Answer 1

$a_{n= a_{1} }+(n-1)*d \\ 5= a_{1}+2d \\ 11= a_{1}+5d$
rezolvind acest sistem obtinem
[tex]6=3d;d=2 \\ a_{1}=1 [/tex]
$a_{9}= a_{1}+8d=1+16=17$