a4= a1+3r ->a1+3r=7 (1) si a1=1 (2) (1) si (2) -relatiile gasite
Inmultim relatia (2) cu nr-ul (-1) ->Si adunam la prima => -(2)+(1)=
-a1+a1+3r=7 -1 -> 3r=6 ->r=2 ->>>>a1= 1 si r=2 ->>a4=a1+3r=1+6=7 (Adevarat) a2015=a1+2014r ->a2015= 1+2014*2 = 1+ 4028=4029