pt n=1
a1=8-6=2
ptn=2
a1+a1+r=2a1+r=8*4-6*2=32-12=20
2*2+r=20
r=16
Extra
verificam pt S3
2+2+16+2+32= 8*9-6*3
20+34=72-18
54=54 adevarat
pt verificare Sn observam ca
a1=2
r=8a1
Sn= na1+(n-1)*n/2 *r
=2n+(n-1) *n*16/2
=2n+ 8 n(n-1)
=2n+8n²-8n
=8n²-6n
deci se verifica ∀n
