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

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Bill Tubbs
Bill Tubbs 2023-1-29
评论： Paul 2025-10-4，14:25
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!

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采纳的回答

Paul
Paul 2023-1-29
Ran in:
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 = 
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Paul
Paul 2025-10-4，14:25
Ran in:
As of R2025A, solve can solve equations for symbolic functions and derivatives:
syms x(t) a
eqn1 = x(t) == a + x(t)/2;
solve(eqn1,x(t))
ans = 
eqn1 = x(t) == a + diff(x(t),t,2)/2;
solve(eqn1,diff(x(t),t,2))
ans = 

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