[MATLAB基础] subs 为什么会失效

[juse12346789]

各位帮我看看
程序

clear
clc
syms tun
x=[0.33
0.264
0.198
0.132
0.066132
-0.066
-0.132
-0.198
-0.2508
-0.264
-0.297
-0.33
-0.297
-0.264
-0.2508
-0.198
-0.132
-0.066
0.066132
0.132
0.198
0.264
0.33];
y=[0
0.19404
0.26281
0.3005
0.32003
0.3203
0.29971
0.26136
0.21549
0.19496
0.13761
0
-0.13952
-0.19622
-0.21925
-0.26248
-0.30162
-0.32281
-0.32373
-0.30373
-0.26314
-0.20216
0];
n=23;tun_ply=0.53/1000;Edb=10.3e+9;Eun=39.0e+9;tdb=4*tun_ply;
for i=1:1:n-1
dL(i,1)=sqrt((x(i+1)-x(i))^2+(y(i+1)-y(i))^2);
end
dA1=dL*tdb/2;dA2=dL*tun;dA3=dL*tdb/2;
for i=1:1:n-1
Sx1(i,1)=dA1(i,1)*dL(i);
Sx2(i,1)=dA2(i,1)*dL(i);
Sx3(i,1)=dA3(i,1)*(y(i)/2+y(i+1)/2-(3*tdb/4+tun)*(x(i)-x(i+1))/dL(i));
end
ESx=sum(Sx1*Edb+Sx2*Eun+Sx3*Edb);
subs(ESx,tun,1)

ans=NaN;
i=1:1:18 或 i=19:1:n-1 切都可以算出数值了
谁能告诉我怎么解决啊