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# How to solve a system of equations symbolically? (MATLAB R2011a)

Asked by Dan on 30 Nov 2012

This is what I'm trying to do:

`    syms x1 x2 x3 X1 X2 X3 t`
```    S = solve('x1 = 3*t*X1+X2', 'x2 = 2*t^2*X2', 'x3 = 5*t*X3')
S = [S.X1 S.X2 S.X3]```

I get a reference to non-existent field error though.

It's solving for x1, x2, and x3, but need to solve for X1, X2, and X3 in terms of x1, x2, x3 and t.

## Products

Answer by Azzi Abdelmalek on 30 Nov 2012
Edited by Azzi Abdelmalek on 30 Nov 2012
```    syms x1 x2 x3 X1 X2 X3 t
S = solve(x1==3*t*X1+X2, x2==2*t^2*X2, x3==5*t*X3,X1,X2,X3)
S = [S.X1 S.X2 S.X3]```

Walter Roberson on 30 Nov 2012

R2011a did not support using == in symbolic expressions. I think it was R2011b that first supported that.

```S = solve('x1=3*t*X1+X2', 'x2=2*t^2*X2', 'x3=5*t*X3','X1','X2','X3')
```
Azzi Abdelmalek on 30 Nov 2012

Then no need to use syms

Walter Roberson on 1 Dec 2012

Yes, correct. If everything is quoted, syms is not needed.