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

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!

Answer 1

[Paul]

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]

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]

That's exactly what I wanted. "isolate" is the right word!

[Paul]

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 =