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# Using solve to solve an equation

Asked by Muthu on 31 Jan 2013

Hi everyone,

I am using 'solve' to solve an equation and I have to solve it for the variable x2. Below is the lines I have in my code.

```                syms x2
solve( d==(x2-x1)^2 + (y2-y1)^2)```

But I get the following error message. "Conversion to double from sym is not possible."

I know that it returns symbolic variable which I have to convert to double I guess when I use x2. So I tried using double(x2)

``` "DOUBLE cannot convert the input expression into a double
array. If the input expression contains a symbolic variable, use the VPA function instead."```

as well as vpa(x2) as it suggested. But I still got the initial error. Any help would be appreciated. Thanks in advance

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Answer by proecsm on 31 Jan 2013
Edited by proecsm on 1 Feb 2013
```syms x1 x2 y1 y2 d
>> solve((x2-x1)^2+(y2-y1)^2-d,x2)
```
```ans =
```
```   x1 - (- y1^2 + 2*y1*y2 - y2^2 + d)^(1/2)
x1 + (2*y1*y2 - y1^2 - y2^2 + d)^(1/2)```

======== edit=============

```subs(ans,{x1,y1,y2,d},[1,2,3,4])
```
```ans =
```
```     -0.7321
2.7321```

Muthu on 31 Jan 2013

hi,

All the values are known except 'x2'. I need to access the value of 'x2' but I am not able to. Any further help is greatly appreciated.

proecsm on 1 Feb 2013

Walter Roberson on 1 Feb 2013

Or

```%set the variables and then,
```
```syms x2
solve((x2-x1)^2+(y2-y1)^2-d,x2)
```
Answer by Walter Roberson on 31 Jan 2013

You are probably using a MATLAB version older than R2011b (I think it was), as the ability to use "==" in symbolic expressions is relatively new. proecsm shows the adjustment you can make to avoid using "=="