c语言求解四阶龙格库塔法的算法源代码，例子：某一地区的病菌传染，三种人员的人数的状态方程，即可能受传染的人数x1,已被传染的病的人数x2，及已治愈的人数x3，并假设他们是因接触而传染。-c language for solving fourth-order Runge-Kutta method algorithm source code, example: a certain area of the bacteria infection, the number three state equation, which may be the number of infected x1, has been the number of disease-borne x2, and the number of people have been successfully cured x3, and assuming they are due to exposure and infection.