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