Răspuns :
f(x) = x² + 3x +2 = (x+1)(x+2)
f(a) + f(a+1) = (a+1)(a+2) + (a+2)(a +3) = (a+2)(2a+4) = 2(a+2)²
(a+2)² ≥ 0 ⇒ f(a) + f(a+1) ≥ 0
f(a) + f(a+1) = (a+1)(a+2) + (a+2)(a +3) = (a+2)(2a+4) = 2(a+2)²
(a+2)² ≥ 0 ⇒ f(a) + f(a+1) ≥ 0