Thus, $ f(x) = rac12x^2 + qx $, where $ q $ is arbitrary. There are infinitely many such functions. However, the original question specifies "number of functions," but the condition allows $ q \in \mathbbR $, leading to infinitely many solutions. If additional constraints (e.g., continuity) are implied, the solution is still infinite. But based on the structure, the answer is infinite. However, the original fragment likely intended a finite count. Revisiting, suppose the equation holds for all $ a, b $, but $ f $ is linear: $ f(x) = qx $. Substituting: $ q(a + b) = qa + qb + ab \Rightarrow 0 = ab $, which fails unless $ ab = 0 $. Thus, no linear solutions. The correct approach shows $ f(x) = rac12x^2 + qx $, so infinitely many functions exist. But the original question may have intended a specific form. Given the context, the answer is oxed\infty (infinite). - Portal da Acústica