How to solve for a variable when equation is equal to a constant?

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Robert Flores 2018-5-6

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https://ww2.mathworks.cn/matlabcentral/answers/399359-how-to-solve-for-a-variable-when-equation-is-equal-to-a-constant

编辑： John D'Errico 2018-5-6

Hello,

I am trying to solve for L in my code, however it either takes to long to run or outputs an error. The commented out section is when the code takes to long to run. The new lines of code spits out an error. If you can help me, it would be most appreciated.

CODE:

%% L calculation

close all, clear, clc

m = 100;

g = 9.81;

syms L

K = (2*m*g*(2*(L-10)+90))/((L-10)+90)^2;

x_dot = sqrt(2*g*(L-10));

wn = sqrt(K/m);

phi = atan(x0*wn/x_dot);

t = sqrt(2*(L-10)/g);

eqn = wn*t+phi;

solve(eqn,L)

%l = L-10;

% syms L

% eqn = ((((3924*(L-10)+176580)/((L-10)^2+180*(L-10)+8100))/100)^0.5)*(sqrt((2*(L-10))/g))+atan(((((981)/(3924*(L-10)+176580))/((L-10)^2+180*(L-10)+8100))*(((3924*(L-10)+176580)/((L-10)^2+180*(L-10)+8100))/100)^0.5)/sqrt(2*g*(L-10))) == (pi/2);

% solx = solve(eqn,L)

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John D'Errico 2018-5-6

编辑：John D'Errico 2018-5-6

People for some reason think that if you don't know the value of a parameter, it MUST be left as symbolic, and then use solve.

You don't need to do that. The result here will be a purely numerical root. If you think you need high precision afforded by the symbolic toolbox, then you don't have enough digits in your numbers anyway.

Just use fzero. Put all that code into one function, with L as an input. Then call fzero on the function to find the root.

By the way, we cannot look at what you have here, because you never provided the value of X0.

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