Using syms, after finding diff(x,y) how to insert initial condition into diff's

So, I'm relatively new to using syms in matlab and am stuck, I have the equation and its partials solved, now I need to insert initial conditions of u1 = 0 into the solved partial derivatives. I tried doing f@ u1 = 0, to solve the function with the idea that I could take the diff's of the solved function over again but matlab is throwing an error with that approach.

syms x1 x2 u1 s1

f = 9*x1^2 - 18*x1*x2 +13*x2^2 - 4 ... 
    + u1*(x1^2 + x2^2 + 2*x1 - 16 + s1^2);
diff(f, x1)
diff(f, x2)
diff(f, u1)
diff(f, s1)

f@ u1 = 0

Answer 1

[Behzad Eydiyoon]

syms x1 x2 u1 s1
f = 9*x1^2 - 18*x1*x2 +13*x2^2 - 4 ... 
    + u1*(x1^2 + x2^2 + 2*x1 - 16 + s1^2);
A=diff(f, x1);
B=diff(f, x2);
C=diff(f, u1);
D=diff(f, s1);
u1=0;
eval(A)
eval(B)
eval(C)
eval(D)