Need Help to correct my this code for the differential equation, Thanks

I get the Answer Zero

Question is = > f(x)= sin(x)+2cos(y)

Need to know that by putting the different value of x and y in the same code

>> syms x y

>> f=inline('sin(x)+2*cos(y)','x',' y')

f =

Inline function:

f(x, y) = sin(x)+2*cos(y)

>> derivative=inline(diff(f(x,y),x,y),'x','y')

derivative =

Inline function:

derivative(x,y) = 0.0

>> derivative(2,5)

[darova]

I don't understand the question. What are you trying to solve