There are two problems in the file that i see. First, the definition of kN should be 1000*N (in the posted file it is 100*N). The second problem is that you changed the definition of the sin(x) function. Use the symbollic equals sign (ctrl=) instead of the := to manipulate equations.
So my mistake was not in defining the variable "b" but it was in modifying the sin function where I was just trying to show my work. The Boolean equal does not resolve an answer unless symbolics is used. Thank you very much for clearing that up.