is n!*n! somehow related to (2n)!?

thanks!

To clarify:

I'm trying to prove that (sorry for the bad formatting...)

integral(cos(x*sin(theta)), theta, 0, pi/2) = c*J0(x)

J0(x) is a bessel function of order zero.

What I did:

I turned the integral, or more specifically, the cos expression into the taylor series expression and also turned J0(x) into a series expression and am trying to cancel both sides. If n!*n! is equal to (2n)! or is somehow related by a constant or something it might be helpful.

Otherwise, if you all can think of a more elegant way to prove it, I'm open to suggestions!