MATLAB做logistics人口模型，求出值有问题
t=1979:2010;
x=[31.26,32.09,33.39,35.45,40.52,43.52,47.86,51.45,55.6,60.14,64.82,68.65,73.22,80.22,...
87.69,93.97,99.16,103.38,109.46,114.6,119.85,124.92,132.04,139.45,150.93,165.13,181.93,196.83,212.38,228.07,241.45,251.03];
f=@(b,t)b(2).*b(3)./(b(3)+(b(2)-b(3)).*exp(-b(1).*(t-1979)));
[b1,r1]=nlinfit(t(1:32),x(1:32),f,[0.03,400,31.26])

Warning: Rank deficient, rank = 2, tol =  2.345153e-11.
> In nlinfit>LMfit at 481
  In nlinfit at 207
Warning: Some columns of the Jacobian are effectively zero at the solution,
indicating that the model is insensitive to some of its parameters.  That may be
because those parameters are not present in the model, or otherwise do not
affect the predicted values.  It may also be due to numerical underflow in the
model function, which can sometimes be avoided by choosing better initial
parameter values, or by rescaling or recentering.  Parameter estimates may be
unreliable.
> In nlinfit at 280

b1 =

   1.0e+11 *

    0.0000    1.4428    0.0000
求出来的b1 b2 b3 值 严重不符 求问错在哪里？