la primul
a₂=a₁+(2-1)r 6=a₁+r / ori(-1) -6=-a₁-r
a₃=a₁+(3-1)r => 5=a₁+2r => 5= a₁+2r adun sistemul
-1=r
daca r=-1 si 6=a₁+r=>6=a₁-1=>a₁=6+1=>a₁=7
a₆=a₁+(6-1)r
a₆=7+5 ori (-1)
a₆=7-5
a₆=2
la celalalt...nu sunt chiar sigura
a₃=a₁+(3-1)r 5=a₁+2r/ ori (-1)=> -5=-a₁-2r
a₅=a₁+(5-1)r=> 11=a₁+4r 11= a₁+4r adun sistemul
6=2r=>r=6:2=>r=3
daca r=3=> 5=a₁+2ori 3
5=a₁+ 6=>a₁=5-1=>a₁=-1
a₇=a₁+(7-1)r
a₇=-1+6r
a₇=-1+6ori3
a₇=-1+18
a₇=17
s₇=(a₁+a₇) ori 7/2
s₇=(-1+17)ori 7/2
s₇=16 ori 7/2
s₇=8 ori 7
s₇=56
