Why won't evalf give me a number.  I have solved for a variable "iDS" higher up in my sheet, and want the approximate value of an element in a matrix.  this element is a function of iDS and no matter what I do, I get the function of iDS!

I really want to use Maple.  I see what it can do, but I often get "stuck" and very frustrated when I can't get it to do what I need it to do.  Sometimes when you are doing assignments, you need a number!