>> x=[2001 2002 2003 2004 2005];
>> y=[500.6 442.4 428.6 370.1 343.1];
>> n=2; % polynomial order
>> p=polyfit(x,y,n) ;
Warning: Polynomial is badly conditioned. Remove repeated data points
or try centering and scaling as described in HELP POLYFIT.
> In polyfit at 79

你不用2次拟合就行，线性就满足要求啦！使用2次的反而是范数增大！