fiminbnd((FUN,x1,x2))可以把解限定在x1和x2之间
FMINBND Scalar bounded nonlinear function minimization.
X = FMINBND(FUN,x1,x2) starts at X0 and finds a local minimizer X of the
function FUN in the interval x1 <= X <= x2. FUN accepts scalar input X and returns
a scalar function value F evaluated at X.