MATLAB: Trying to Create a Riemann Sum function from scratch, keep getting function error.

errorfunctionriemannsum

So i have to create a riemannsum calculator function, its been a little bit since ive used MatLab, but this seems right i just dont understand why i keep getting this error.
function [ Result ] = RiemannSum(Fun,a,b) %This function will calculate the riemann sum of a function defined from an %interval from a to b. This will be done using 10 subintervals.
dx=(b-a)/10;
XS=(a+(dx/2):dx:b-(dx/2));
Result=sum((feval(Fun,XS)).*dx);
The error i get is : RiemannSum(2*x,1,10) Undefined function or variable 'x'.
Even if i set x=XS it doesnt fix the error, im really stuck here.
Or when i tinker around and first run x=[] ill get this error instead.
Error using feval Argument must contain a string or function_handle.
Error in RiemannSum (line 6) Result=sum((feval(Fun,XS))*dx); end

Best Answer

Please type the following at the MATLAB Command Prompt:
   F = @(x) 2*x;
   R = RiemannSum(F,1,10);
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