Răspuns:
Explicație pas cu pas:
Notez S=100+200+300+....+1200-25-50-75-....-300
S=100+200+300+....+1200-25-50-75-....-300=
=100·(1+2+3+...+12)-25·(1+2+3+...+12)=(100-25)·(1+2+3+...+12)=75·(1+2+3+...+12) =>
S/75=1+2+3+...+10+11+12
S/75=12+11+10+...+3+2+1
2·S/75=(1+12)+(2+11)+(3+10)+...+(10+3)+(11+2)+(12+1) =>
2·S/75=13+13+13+...+13+13+13=13·12 => S=13·12·75/2=13·6·75=5850
