Empty sym: 0-by-1

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Delia Bosshart 2021-5-24

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https://ww2.mathworks.cn/matlabcentral/answers/838463-empty-sym-0-by-1

评论： Star Strider 2021-5-26

I don't understand why matlab gives as an output "Empty sym: 0-by-1" to the following equation which I want to solve. I have used this equation many times with other numbers and it has always worked since. Thank you for the help!

syms x1

eqn = (10.5145 - 0.5 + 6.96.*x1 + (6-(31*(232.87.*(x1*12).^(-0.46)))/1000).*x1/60*1000 + 31.*(232.87.*(x1*12).^(-0.46))/1000.*0/60*1000)*0.0086 - 0.1475 - x1 == 0

vpasolve(eqn, x1)

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Answer 1

Star Strider 2021-5-24

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https://ww2.mathworks.cn/matlabcentral/answers/838463-empty-sym-0-by-1#answer_707753

在 MATLAB Online 中打开

Exploring the function is often worthwhile. It apparently has a real root at about -0.45, so share that information with vpasolve:

syms x1

eqn = (10.5145 - 0.5 + 6.96.*x1 + (6-(31*(232.87.*(x1*12)^(-0.46)))/1000)*x1/60*1000 + 31*(232.87*(x1*12)^(-0.46))/1000*0/60*1000)*0.0086 - 0.1475 - x1 == 0;

f(x1) = (10.5145 - 0.5 + 6.96.*x1 + (6-(31*(232.87.*(x1*12)^(-0.46)))/1000)*x1/60*1000 + 31*(232.87*(x1*12)^(-0.46))/1000*0/60*1000)*0.0086 - 0.1475 - x1;

x1_s = vpasolve(real(f), -0.5)

x1_s =

figure

fplot(real(f), [-1 1])

hold on

fplot(imag(f), [-1 1])

hold off

grid

legend('Real','Imag', 'Location','best')

.

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Delia Bosshart 2021-5-26

Thank you! Therefore the main problem is that I have a negative number in the root and that is why it can't solve the equation right?

Star Strider 2021-5-26

As always, my pleasure!

One problem is that solve identified 50 roots of that function, so there could be a number of them. However plotting it revealed only one .real root. It is a relatively complicated function. I have no idea what the other roots were, since solve did not calculate them. Plotting the real and imaginary parts of the function demonstrated its behaviour and led to the solution.

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