Solving a complex integral

Hi, I have a complex integral to solve, however, there are some weird things here:

syms x h a b c n k
F = cos.^2(n.*pi.*x);
G = cos.^2(n.*pi.*x).*x.^2;
a = i.*h.^3.*diff(F,x,3);
b = -k.*[diff(G,x,3)];
c = 1/4.*k.*x.^4.*(cos.^2(n.*pi.*x));
f = a.*b.*c
result2a = int(f, x)
result2b1 = vpa(result2a)
result2b2 = double(result2a)

However, F = cos.^2(n.*pi.*x); is not accepted, why?

Answer 1

[Birdman]

F = cos(n.*pi.*x).^2;

and the same goes for other cos terms.

[Sergio Manzetti]

Indeed, thanks!