# How to simply rearrange a symbolic equation to get an expression for one variable in terms of others

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Bill Tubbs on 29 Jan 2023
Commented: Bill Tubbs on 29 Jan 2023
I'm building differential equations symbolically and ran into an error when doing a simple re-arranging of an equation.
For example, I am able to rearrange an equation with a symbolic variable x:
syms x a
eqn1 = x == a + x/2;
% Re-arrange to get an expression for x
solve(eqn1, x)
ans =
2*a
But when x(t) is time-varying, solve does something else:
syms x(t) a
eqn1 = x(t) == a + x(t)/2;
% Re-arrange to get an expression for x(t)
solve(eqn1, x(t))
Warning: Unable to find explicit solution. For options, see help.
> In sym/solve (line 317)
ans =
struct with fields:
a: [0×1 sym]
t: [0×1 sym]
I'm not sure what it's trying to do. All I want is to re-arrange the expression!

Paul on 29 Jan 2023
Not sure about solve, but isolate seems to work
syms x(t) a
eqn1 = x(t) == a + x(t)/2;
isolate(eqn1,x(t))
ans =
Walter Roberson on 29 Jan 2023
This is probably the easiest route.
solve() is not able to solve for anything other than variables, so the approach with solve() would have to be to subs() a variable for the function, solve for the variable, and then if needed subs() back.
Bill Tubbs on 29 Jan 2023
That's exactly what I wanted. "isolate" is the right word!

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