S2=a1+a1+r=2a1+r
S3=a1+(a1+r)+(a1+2r)=3a1+3r
S4=a1+(a1+r)+(a1+2r)+(a1+3r)=4a1+6r
inlocuim in sistemul nostru
(2a1+r)-(4a1+6r)+(a1+r)=14
3a1+3r+a1+2r=17
-a1-4r=14
4a1+5r=17
-4a1-16r=56
4a1+5r=17
--------------------------
-11r=73⇒r=-73/11
a1=-14+292/11=(-154+292)/11=138/11
