The well-known economic principle on profit states that the profit is maximum when the marginal revenue equals the marginal cost. We hereby present the case where the price and the cost are polynomials in the demand quantity units. The coefficients are trapezoidal fuzzy numbers, hence the price and the cost are fuzzy numbers too. Since our goal is maximizing the profit, we have to choose a suitable defuzzification method. We use the Weighted Average Value one, which is more general than others presented by several authors. The results we obtain are a gene-ralization of the crisp case.