我这里提示：
未定义函数或变量 'fl'。
不过从你的代码来看，你最终要求得的方程是这个：
rx =(r0 + r1.*x +r2.*x.^2)/(1+r3.*x+r4.*x.^2) == 0
这个方程的 r1 r2... r4 仍然是符号变量，如果你要将前面求得的解代入的话，你需要在求解完
[r0,r1,r2,r3,r4] = solve(s1 == 0,s2 == 0,s3 == 0,s4 == 0,s5 == 0)
之后，再赋值：
rx =(r0 + r1.*x +r2.*x.^2)/(1+r3.*x+r4.*x.^2);
然后求解：
tr = vpa(solve(rx == 0),6)
clear;
clc;
l = 0 ;
m = 1 ;
n = 0.2406;
syms r0 r1 r2 r3 r4 x tr;
rx =(r0 + r1.*x +r2.*x.^2)/(1+r3.*x+r4.*x.^2);
drx = diff(rx);
f = (x - 1/4).* (2 - x).* (x + 5).^2;
s1 = subs(rx,x,l)-fl;
s2 = subs(rx,x,m)-fm;
s3 = vpa(subs(rx,x,n) -fn,6);
s4 = subs(drx,x,l) - dl;
s5 = vpa(subs(drx,x,n) - dn,6);
[r0,r1,r2,r3,r4] = solve(s1 == 0,s2 == 0,s3 == 0,s4 == 0,s5 == 0) ;
rx =(r0 + r1.*x +r2.*x.^2)/(1+r3.*x+r4.*x.^2);
tr = vpa(solve(rx == 0),6) ;
