Question

cum se face progresia asta ?

Answer 1

a3=a1+2r
a1+2r=5

a6=a1+5r
a1+5r=11

Si formam sistemul:
{a1+2r=5
{a1+5r=11
Scadem cele doua relatii:
a1-a1+2r-5r=5-11
-3r=-6
r=2
Si acum putem afla a1:
a1+4=5 => a1=1

a9=a1+8r=1+8*2=1+16=17