Question

Determinati nr. x incat 1+2+....+x=66

Answer 1

x(x+1)/2 = 66
x^2 + x = 132
x^2 + x - 132 = 0
delta = 1+4•132=1+528=529 => radical din delta = 23
x1 = (-1-23)/2 = -12 Nu e bun
x2 = (-1+23)/2 = 11

Verificare : 1+2+3+...+11 = 11•12/2 = 66