Let $f:\mathbb{R}\to\mathbb{R}$ be a twice differentiable function such that $f''(x)>0$ for all $x\in\mathbb{R}$ and $f'(a-1)=0$, where $a$ is a real number. Let $g(x)=f(\tan^2x-2\tan x+a)$, $0<x<\dfrac{\pi}{2}$.
Consider the following two statements:
(I) $g$ is increasing in $\left(0,\dfrac{\pi}{4}\right)$
(II) $g$ is decreasing in $\left(\dfrac{\pi}{4},\dfrac{\pi}{2}\right)$. Then,