Error using sym/subsindex (line 796)

Question

Nate Mahorner 2017-10-19

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/362222-error-using-sym-subsindex-line-796

评论： Star Strider 2017-10-19

t1 = 588; %k 
do = 3.5; % in
di = 3.0; % in
kp = 21.6; %W/(m*K)
ta = 294; % K
ts = 255.4; %k
h = 48.3; %W/(m^2*K)
deltar = 1; % in of calcium silicate insulation
e = .76;
at = pi*(do+2*deltar);
alm1 = (pi*(do-di))/(log(do/di));
alm2 = (2*pi*deltar)/log(do+2*deltar/do);
o = .0000000567; %W/(M^2*k^4)
d = .5; %do-di(in)
syms t2 t3 ki
eq1 = ki==.044+.0001144*(.5*(t2+t3)-283.333)
eq2 = 1/((d/(2*kp*alm1))+(deltar/ki*alm2))==(h*at(t3-ta)+e*o*at(t3^4-ts^4))/(t1-t3)
eq3 = (2*kp*alm1*(t1-t2))/((d)==h*at(t3-ta)+e*o*at(t3^4-ts^4))
[t2,t3,ki] = vpasolve([eq1,eq2,eq3],[t2,t3,ki],[0 inf;0 1])
t2 = double(t2)
t3 = double(t3)
ki = double(ki)

the error is in the eq2 line, eq1 seems to work fine.

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Nate Mahorner 2017-10-19

@star strider I figured it out right before you answered! Thank you so much for the quick help!

Star Strider 2017-10-19

The mystery nevertheless continues, since ‘s’ — either as a function or an array — appears nowhere in your code (that I can see).

Resolved it seems, regardless.

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Answer 1

Birdman 2017-10-19

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/362222-error-using-sym-subsindex-line-796#answer_286717

You forgot to put an operator in front of your at variables in eq2 and eq3. Also, in the vpasolve function, choose your initial guess wisely. If you don't indicate an initial guess for your equations, MATLAB still finds answers. Hope this helps.