solving two equations with 4 unknowns

Hello all,

Can you please tell me how can I solve 4 equations with 2 unknowns in MATLAB?

[Sean de Wolski]

Are they symbolic or numeric?

[Sean de Wolski]

How is this urgent? Is it going to cure Cancer, end world hunger/tyranny, get us out of work early on this gorgeous Thursday, anything?

[Bahareh]

It is symbolic.

Answer 1

[Sean de Wolski]

Edited: Walk before you run:

syms x y
results = solve('3*x+y=9','x-7*y=-3',x,y)
results.x
results.y

look at

doc solve

to learn more.

[Bahareh]

Thanks, I'm gonna try it now.

[Bahareh]

Sorry to bother you, before I use the solve command I did the following:

syms T12 T11 T21 T22 R11 R12 R21 R22 H21_1 H21_2 H12_1 H12_2 lambda beta alpha

>> (T21.^2.* H21_1.^2+beta^(-1) ).* R11.^2+(T21.* T2_2.* H21_1.* H21_2 ) .*R11.* R12=(lambda*alpha).*((R21.^2.* H12_1.^2+lambda).* T11.^2+(R21.* R22 H12_1.* H12_2 ).* T11.* T12 );

??? (T21.^2.* H21_1.^2+beta^(-1) ).* R11.^2+(T21.* T2_2.* H21_1.* H21_2 ) .*R11.* R12=(lambda*alpha).*((R21.^2.* H12_1.^2+lambda).* T11.^2+(R21.* R22 H12_1.* H12_2 ).* T11.* T12 );

|

Error: The expression to the left of the equals sign is not a valid target for an assignment.

How can I deal with this error?

[Sean de Wolski]

First off, you don't have a valid variable T2_2. Second, to see if they're equal use '==' as opposed to '=' - an equality test as opposed to an assignment. Third, asking if they're equal like that will surely fail because not all variables are on both sides.

Fourth, to solve you input the whole thing into the function solve and then tell it what variable to solve for.

[Bahareh]

Thanks.

[Bahareh]

can you please explain your last sentence more?

Answer 2

[Walter Roberson]

syms T12 T11 T21 T22 R11 R12 R21 R22 H21_1 H21_2 H12_1 H12_2 lambda beta alpha 
solve((T21.^2.* H21_1.^2+beta^(-1) ).* R11.^2+(T21.* T2_2.* H21_1.* H21_2 ) .*R11.* R12 - (lambda*alpha).*((R21.^2.* H12_1.^2+lambda).* T11.^2+(R21.* R22 H12_1.* H12_2 ).* T11.* T12 ), lambda)

Notice that the A=B form you had has been replaced by A-B, which solve will interpret as indicating that (A-B)=0 is desired.

In the second line, replace the "lambda" at the end by the name of the variable you want to solve for. As you only have one equation, you can only effectively solve for a single variable.

Answer 3

[SooShiant]

Combine 2 equation to gain 1 then use this way:

Here is a simple example which you can change the equation and range and solve yours. The equation is 12x+9y+7z-60=0 where x,y,z are integers varies 0 to 10:

x=[0:1:10];
y=[0:1:10];
z=[0:1:10];
[X,Y,Z]=ndgrid(x,y,z);
F=12.*X+9.*Y+7.*Z-60;
idx=find(F==0);
[X(idx(:)),Y(idx(:)),Z(idx(:))];

Equations of this type are known as Diophantine equations.