说明：  matlab最小二乘原理下的曲线拟合方法汇总：polyfit函数、“\”命令、fminsearch函数、nlinfit函数。附上一个具体的例子：1900-2000年的总人口情况的曲线拟合。
(matlab principle of least squares curve fitting method Summary: polyfit function, " \" command, fminsearch a function, nlinfit function. Attach a specific example from 1900 to 2000, the total population of the curve fitting.)