Individual fuzzy membership values of a crisp number

Question

Consider a trapezoidal membership function having values a, b, c, d. How can I define fuzzy membership function for four linguistic variables like {low, medium-low, medium-large, large} with a defined equation with all these four variables? In the case case of a triangular membership function with (a, b, c) I can define membership function for μ(low) as Max{0,min((b-x)/(b-a) , 1)} and μ(medium) as Max{0,min((x-a)/(b-a) , (c-x)/(c-b))} and μ(high) as Max{0,min((x-b)/(c-b) , 1)}.