Hi Abhijit,
Here you don't need to use index. I think you are trying to create a function of x with f(x), but as you have already defined absolute value of x, it is being taken as index and in matlab indices are positive integers. Your code will work without index for f.
x=0.05:0.05:1;
y=log(x)';
f=x.*y;
[fMin,idx]=min(f);
result=x(idx);
If you want to first create a function of x and then you need to first declare it using sym/syms. To know more about how to create symbolic functions follow this link: https://in.mathworks.com/help/symbolic/create-symbolic-functions.html
Cheers
