MATLAB不提供现成指数拟合函数，一方面由于但可以轻松通过多项式拟合转化而来，但是使用上还是不如有一个函数方便，所以特写这样一样简单实用的函数-MATLAB does not provide ready-exponential fit function, by hand because it can be easily transformed　a polynomial fit, but still not as good as there is easy to use a function, it is as simple close-up of such useful functions