Problems with partial derivatives

I need to calculate the partial derivative of a function with respect to a scalar, but diff(a,e) does not work (if a is my function and e my scalar). It always tells me that N must be a positive integer.

now this is my function: a = e*sin(alpha)/(sin(alpha+betha))

and this is the scalar: e = 112.376

first i thought i'd have to create symbolic expressions for all sin and alpha etc. but those are obviously no symbolic expressions but variables with values. So I would need to get a value as a result from the partial derivative. Or do i need to create all of those variables as symbolic expressions first, then see what function matlab will give me as a result and then plug the values back in?