Aloha :)
$$J_n(x)=\frac1\pi\int\limits_0^\pi\cos\left(x\sin t-nt\right)dt$$$$J'_n(x)=\frac1\pi\int\limits_0^\pi\frac{\partial}{\partial x}\cos\left(x\sin t-nt\right)dt=-\frac1\pi\int\limits_0^\pi\sin\left(x\sin t-nt\right)\sin t\,dt$$$$J''_n(x)=-\frac1\pi\int\limits_0^\pi\frac{\partial}{\partial x}\sin\left(x\sin t-nt\right)\sin t\,dt=-\frac1\pi\int\limits_0^\pi\cos\left(x\sin t-nt\right)\sin^2t\,dt$$