S=4+7+10+...+91=
pasul 7-4=3
nr termeni (91-4):3+1=29+1=30
=4+4+3+4+6+....+4+87=4*30+3(1+2+...+29)=120+3*29*30/2=120+1305=
=1425
S=6+11+16+12+...+101=
pasul 11-6=5
nr termeni (101-6):5+1=19+1=20
=6+6+5+6+10+....+6+95=6*20+5(1+2+...+19)=120+5*19*20/2=120+950=
=1070
