6. (6 marks)
Find the second derivative of $f(x) = \frac{\sin(bx)}{1 + \cos(bx)}$ where $b$ is a parameter,
and calculate its value at $x = 0$, i.e. $f''(0)$, as an algebraic expression of $b$.
(HINT: simplify the first derivative $f'(x)$ before calculating $f''(x)$.)