f(1)=2*1-5=2-5=-3 f(2)=2*2-5=-1 f(3)=2*3-5=1 f(4)=2*4-5=8-5=3 ............... f(100)=100*2-5=200-5=195 f(1)+f(2)+...f(100)= -3+(-1)+1+3+...+195 Izolam termenii cu minus 1+3+...+195. Gauss pt numere impare, adica 1+3+5+...+2n-1=n*n 2n-1=195 2n=196 n=98 deci suma numerelor pozitive fa fi egala cu 98*98 Faca adunam si numerele negative, vom avea  -3+(-1)+98*98 =  -3-1+9604 =  =9600