how to tune the parameters of the trimf grad function?

I want to tune the parameters(a,b,c) of the trimf grad function with steepest descent method,but due to grad function is a segmentation function, when a=<x<=b c is not exist and a is not exist when b=<x<=c,i can't get the gradient of the parameter c when a=<x<=b and the gradient of the parameter a when b=<x<=c,do you have better method to solve the problem? thank you advanced.