If y=f(\cosh x) and f^{'}(x)=\log (x+\sqrt{x^2-1}), then \frac{d^2 y}{d x^2}=

Correct Answer is : sinhx+xcoshx

{x (2 \sqrt{x^2-1}+1) }/{\sqrt{x^2-1} (x^2+\sqrt{x^2-1}) }