Error during integration after differentiation

%%
f = @(x,y,z) x.*y.^3.*z.^3; % define the input function
syms x;
g =diff(f,x)
Q = integral3(g,0,2,0,2,0,2) % LHS of divergence theorem


Invalid argument at position 1. First input argument must be a function handle.

Any one can help above , as i differeniate a function g and then would like to integate it , but it show without function handle

Answer 1

[Dyuman Joshi]

When you declare x as a symbolic variable, g will defined a symbolic variable as well. And as the error states, integral3 requires the input to be a function handle (which g is not)

f = @(x,y,z) x.*y.^3.*z.^3; % define the input function
syms x y z

g = diff(f,x)

g = 

class(g)

ans = 'sym'

You can integrate like this

%y and z should be syms variable as well to use int()
val = double(int(int(int(g,x,0,2),y,0,2),z,0,2))

val = 32

%verifying
h = @(x,y,z) y.^3.*z.^3;
q = integral3(h,0,2,0,2,0,2)

q = 32.0000

P.S - using matlabFunction will give a different answer, so you won't get the desired result with it and integral3()

G=matlabFunction(g)

G = function_handle with value:
    @(y,z)y.^3.*z.^3

Answer 2

[Askic V]

Perhaps, this is what you asked for:

f = @(x,y,z) x.*y.^3.*z.^3; % define the input function
syms x;
g = diff(f,x);
% use eval function
h = @(x,y,z) eval(g)

h = function_handle with value:
    @(x,y,z)eval(g)

Q = integral3(h,0,2,0,2,0,2) % LHS of divergence theorem

Q = 32.0000