double sin(double x) { return x; }
However, even if the sin function is already tested elsewhere, you will still need further testing to ensure that you are calling it correctly (e.g. not confusing degrees and radians).
EDIT: Yes, I read it wrong - clearly need coffee...
That said... that kind of coverage isn't quite as useless as it might seem. If your tests do execute every line, even in a completely contrived way, you will catch a lot if you change your code. You just tend to catch more of the "wrong number of arguments passed to a method" kind of error than "you are allowing the autopilot to try to land the plane 100 feet below the runway" kind of error ;)