[求助]如何解方程？

[pslpsl]

 
clc;clear;
syms x1 x2 x3 x4;
%以下代入实际的函数
equ1=f1(x1,x2,x3,x4)-A;
equ2=f2(x1,x2,x3,x4)-B;
equ3=f3(x1,x2,x3,x4)-C;
equ4=f4(x1,x2,x3,x4)-D;
%以上代入实际的函数
[x1,x2,x3,x4]=solve(equ1,equ2,equ3,equ4,x1,x2,x3,x4); 
x1=vpa(x1,5)
x2=vpa(x2,5)
x3=vpa(x3,5)
x4=vpa(x4,5)
%BY PSL 2008 08 21