Why would you go through the trouble of programming a bisection algorithm yourself when Mathcad offers much better algorithms built-in.
Furthermore I am confused why you think you will get 5 roots in the range [0.1; 30]. You will just get one at approx x=1.879 and the function result will go complex soon after (approx x>1.88919).
P.S.: You function seem to fail because in ZBRAK you compare an expression which contains a function value f(x) (fc) against 0. This throws an error if f(x) is not real, which is the case for x-values greater than 1.88919.
