CODEWITHSUNDEEP
matlab
Marcus
Marcus

Reputation: 11

Trying to solve 2 equations with 2 unknowns symbolically using MATLAB

Trying to solve 2 equations with 2 unknown using MATLAB and it is not liking my input.

Where c is 3.06 + 2.57j, and v is 12:

eq1 = 'c = a/10j'
eq2 = '(6b)/50 + b/10j = a/10j + a/10'

Trying to solve for a and b. Any suggestions of how to properly put these in?

Upvotes: 1

Views: 14681

Answers (2)

John Alexiou
John Alexiou

Reputation: 29264

Matlab is screwed up, but the syntax is

sol = solve(eq1,x1,eq2,x2,..);

It would make more sense to make it solve({eq1,eq2,..},{x1,x2,..}) but no, we have to write out all the arguments one by one.

Anyway the trick is that eq1, eq2, .. are symbolic expressions that must be evaluated to zero. So instead of c = a/10j it needs to be eq1=sym('c-a/10j')

So try this:

eq1 = sym('c - a/(10*i)');   % must evaluate to zero
eq2 = sym('(6*b)/50 + b/(10*i) -( a/(10*i) + a/10)'); %must evaluate to zero

sol = solve(eq1,'a',eq2,'b');

a = subs(sol.a,'c',3.06+2.57j)
b = subs(sol.b,'c',3.06+2.57j)

produces a= -25.7000 +30.6000i and b=-20.6639 +29.6967i.

Note that symbolic functions do not understand 10j. It needs to be 10*i. Also the notatation 6b is missing an operator and needs to be 6*b.

Good luck!

Since the problem is linear there is another way to solve it

equs = [eq1;eq2];
vars = [sym('a');sym('b')];
A = jacobian(equs,vars);
b = A*vars - equs;

x = A\b;
c = 3.06+2.57j;
sol = subs(x,'c',c)

with results

sol =
 -25.7000 +30.6000i
 -20.6639 +29.6967i

Upvotes: 2

Marm0t
Marm0t

Reputation: 931

you need to tell matlab that c,eq1,eq2,a,j (is this not complex?), and b are a symbolic variables. i.e. use the command "syms a b c j eq1 eq2". Then define everything as you have done above minus your single quotes (that's a string!).

then you can just use the 'solve(eq2,'variable you want to solve for'). Easy enough.

Upvotes: 0

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