Is there a way to allow students to enter subscripts in their answers?

For example, for Calc 2 volume problems, I would like my students to enter the formula for area of the ith rectangle using subscripts on x or y.

Ex. A_{i} = (x_{i}^{2} - x_{i})*delta x_{i}.

I suppose this also requires them to enter delta x.

It would seem to me that they should be allowed to enter x_i, but I cannot seem to get the context to recognize this as a legal variable.

Perhaps I could have them enter xi for x_{i}, but then it would be nice to have a way for this to be redefined as x_i for the Preview.

Anyone have suggestions for me?

Thanks!

Paul