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.