Question

Se considera suna S=1+2+3+.....+n,n N. determinati numarul natural n daca reprezinta 10% din S

Answer 1

S = n(n+1) / 2; n=10/100*S;  n= 10/100 * n(n+1)/2; n=1/10*n(n+1)/2;  n = n(n+1)/20;
20n=n(n+1);  20n = n² + n;  n²+n-20n =0;  n²-19n=0: Δ=(-19)²  n=19+19totul supra 2
n = 38/2; n=19