要获取数值差(对称差)，请计算(f(x+dx)-f(x-dx))/(2*dx)

To get a numerical difference (symmetric difference), you calculate (f(x+dx)-f(x-dx))/(2*dx)

fx = @(x)x.^2;
fPrimeAt3 = (fx(3.1)-fx(2.9))/0.2;

或者，您可以创建函数值的向量并应用 DIFF ，即>

Alternatively, you can create a vector of function values and apply DIFF, i.e.

xValues = 2:0.1:4;
fValues = fx(xValues);
df = diff(fValues)./0.1; 

请注意，diff采用前向差，并且假定dx等于1.

Note that diff takes the forward difference, and that it assumes that dx equals to 1.

但是，根据您的情况，***将fx定义为多项式，并评估函数的导数，而不是函数值.

However, in your case, you may be better off to define fx as a polynomial, and evaluating the derivative of the function, rather than the function values.