S2

代码说明：

说明：  function [ret,x0,str,ts,xts]=s1(t,x,u,flag) s1 is the M-file description of the SIMULINK system named s1. The block-diagram can be displayed by typing: s1. SYS=s1(T,X,U,FLAG) returns depending on FLAG certain system values given time point, T, current state vector, X, and input vector, U. FLAG is used to indicate the type of output to be returned in SYS. Setting FLAG=1 causes s1 to return state derivatives, FLAG=2 discrete states, FLAG=3 system outputs and FLAG=4 next sample time. For more information and