Find a zero of a function of one variable. 

  FZERO(F,X) finds a zero of f(x).  F is a string containing the

  name of a real-valued function of a single real variable.   X is

  a starting guess.  The value returned is near a point where F

  changes sign.  For example, FZERO('sin',3) is pi.  Note the

  quotes around sin.  Ordinarily, functions are defined in M-files.



  An optional third argument sets the relative tolerance for the

  convergence test.   The presence of an nonzero optional fourth

  argument triggers a printing trace of the steps.