Question

Se considera progresia aritmetica (a indice n) nmai mare sau egal cu 1 in care a3=5 si a6=11. Sa se calculeze a9

Answer 1

a9 = a3 + a6 
a9 = 5 + 11 
a9 = 16 
coroană ?

Answer 2

a3=5--->a1+2r=5
a6=11--->a1+5r=11
a9=?
$\left \{ {{a1+2r=5|*(-1)} \atop {a1+5r=11}} \right. =\ \textgreater \ -2r+5r= -5+11 \ \textless \ =\ \textgreater \ 3r= 6 |:3=\ \textgreater \ r= 2$
Aflam a1:
$a1+2r=5 \ \textless \ =\ \textgreater \ a1+2*2= 5 =\ \textgreater \ a1+4=5 |-4 \ \textless \ =\ \textgreater \ a1=1$
a9=a1+8r
a9=1+8*2
a9=17