Problem with equations solver

Question

Carlo Pezzoli 2018-7-18

0
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https://ww2.mathworks.cn/matlabcentral/answers/410951-problem-with-equations-solver

回答： Areej Varamban Kallan 2018-8-2

Hi all, I've got a problem with the equations solver. I'm sure that my code is wrong (in the function "solve") because I've used symbolic functions, but I don't know where is the mistake.

 e
syms alpha(t) beta(t) c(t)
eqns = [(a*cos(alpha))+(b*cos(beta))==0 , (a*sin(alpha))+(b*sin(beta))==c];
sol = solve(eqns, [c, beta]);  
ALPHA=0:pi/100:4*pi;
plot(ALPHA, subs(sol.c(2,1), alpha(t) ,ALPHA))
eqns_dot=diff(eqns, t)

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Answer 1

Areej Varamban Kallan 2018-8-2

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Hi Carlo,

I understand that you are having issues when using the 'solve' function to solve symbolic equations.

When solving symbolic equations using 'solve(eqn,vars)', 'vars' should be an array of symbolic variables and not symbolic functions. Moreover when using 'plot(x,y)', 'x' and 'y' must be numeric, datetime, duration or an array convertible to double. Here I assume a = 2 and b = 3 and substitute these in sol.c when plotting.

syms a b c alpha(t) beta
eqns = [(a*cos(alpha))+(b*cos(beta))==0 , (a*sin(alpha))+(b*sin(beta))==c];
sol = solve(eqns, [c, beta]); 
ALPHA=0:pi/100:4*pi;
plot(ALPHA, subs(sol.c(2,1), {alpha(t) ,a,b},{ALPHA,2,3}));
eqns_dot=diff(eqns, t);