33.MATLAB编程 方程求根 源程序代码.rar

function root=QBS(f,a,b,x,eps) format long; if(nargin==4) eps=1.0e-6; end fa=subs(sym(f),findsym(sym(f)),a); fb=subs(sym(f),findsym(sym(f)),b); fx=subs(sym(f),findsym(sym(f)),x); if(fa==0) root=a; return; end if(fb==0) root=b; return; end if(fx == 0) root = x; return; else tol=1; t=zeros(3); t(1)=a; t(2)=x; t(3)=b; while(tol>eps) f1=subs(sym(f),findsym(sym(f)),t(1)); %计算三点的函数值 f2=subs(sym(f),findsym(sym(f)),t(2)); f3=subs(sym(f),findsym(sym(f)),t(3)); d1=f2-f1; d2=f3-f2; d3=f3-f1; if (d1 == 0 || d2 == 0 || d3 == 0) disp('除0错误，请重新选择三个初始点！'); root = NaN; return; end root=t(1)*f2*f3/d1/d3+t(2)*f1*f3/d1/d2+t(3)*f2*f1/d2/d3; tol=abs(root-t(3)); ft = subs(sym(f),findsym(sym(f)),root); if( f2*ft < 0) if root > t(2) t(1)=t(2); t(2)=root; else t(3) = t(2); t(2) = root; end else if f2 < 0 if root > t(2) t(1)=t(2); t(2)=root; else t(1) = root; end else if root > t(2) t(3) = root; else t(3) = t(2); t(2) = root; end end end end end format short;