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matlabsymbolic-math

Implicit derivative with Matlab symbols

Is it possible to have MATLAB do implicit differentiation with its symbols? I have the following code

syms x;
y = symfun(sym('y(x)'), sym('x'));
yPrime = symfun(sym('y+(2*x)-1'), [sym('x'), sym('y')]);
diff(yPrime, x, 1)

From this I get

ans(x, y) =
2

but what I want to get (in some form) is 

ans(x, y) =
dy/dx + 2

Is that possible?

Solution

  • It looks you're trying to use y as both a symbolic variable and an abstract symbolic function (symfun), which isn't possible. A symbolic function can only take symbolic variables as arguments. I think that you can accomplish what you want with (old-style strings not needed)

    syms y(x)        % Implicitly defines x too
yPrime = y+2*x-1 % Also a symfun because y is a symfun
diff(yPrime,x)

    which returns this symfun

    ans(x) =

diff(y(x), x) + 2