48+49+...+75-(1+2+3+...47)+(1+2+3+...+47)=(1+2+3+...+75)-(1+2+3+...47)=(75*76:2)-(47*48:2)=2850-1128=1722
53+54+...79=53+54+...+79-(1+2+3+...52)+(1+2+3+...52)=(1+2+3+...79)-(1+2+3+...+52)=(79*80:2)-(52*53:2)=3160-1378=1782
1+3+5+7+...+101=1+2+3+...+(2*50+1)=51²
100x99-99x98+96x97-97x94+95x94-95=99x(100-98)+97x(96-97)+95x(94-1)=99x2+97x3+95x93=198+291+8835=9324
folosim formulele 1+2+3+...+n=n*(n+1):2
1+3+5+...+(2n+1)=(n+1)²
2+4+6+...+4008-(2+6+...4006)=4007+4008=8015
la ultima observam ca avem 2-2,4-4....si tot asa pana raman doar 4007 si 4008
