Here's what you are asking for. Like Tom, I have some real doubts that your equation is correct though. You have "a" on it's own, "a" multiplied by deg, and "a" multiplied by PI/180. That is highly unlikely to be correct. If you want to work in degrees multiply your assigned values by deg (a.0:4.0*deg), put deg in the units placeholder of the results, but do not attempt to do conversions inside the expression.
Richard