Answer by rburing for I have a symbolic function $g(x,y)$, which depends on the variables $x$ and $y$. Using this, I define the function $f(x,y)$ as: \begin{equation} f(x,y) = g(x,y) + 2x. \end{equation}If I calculate the derivative of $f(x,y)$ with respect to $x$: \begin{equation} \frac{df(x,y)}{dx}=\frac{dg(x,y)}{dx}+2. \end{equation}Now, I need to evaluate this at $x=0$, knowing that $\frac{dg(x,y)}{dx}\bigg\rvert_{x=0}=10$. This should give me: \begin{equation} \frac{df(x,y)}{dx}\bigg\rvert_{x = 0} = \frac{dg(x,y)}{dx}\bigg\rvert_{x = 0} +2=12 \end{equation}The code I have written to achieve this is the following:x = var('x') y = var('y') g = function('g')(x,y) #symbolic function f = g + 2*x der_f = diff(f,x); der_f and this is what I get:diff(g(x, y), x) + 2 as I expected. However, I don't know how to follow. In particular, I need to know how to:1) assign $\frac{dg(x,y)}{dx}\bigg\rvert_{x=0}=10$,2) evaluate $\frac{df(x,y)}{dx}$ at $x=0$, so that I obtain $\frac{df(x,y)}{dx}\bigg\rvert_{x = 0} = 12$.

Instead of "assigning" a value to $\partial g/\partial x \vert_{x=0}$ beforehand, it's easier to make the substitution afterward: sage: der_f.subs(x==0).subs(diff(g,x).subs(x==0) == 10) 12 Or in steps, mimicking the order you proposed: sage: what_i_know = diff(g,x).subs(x==0) == 10 sage: der_f.subs(x==0).subs(what_i_know) 12