Question:
Mark the correct alternative in the following:
The function $f(x)=2 \log (x-2)-x^{2}+4 x+1$ increases on the interval.
A. $(1,2)$
B. $(2,3)$
C. $((1,3)$
D. $(2,4)$
Solution:
Formula:- The necessary and sufficient condition for differentiable function defined on $(a, b)$ to be strictly increasing on $(a, b)$ is that $f^{\prime}(x)>0$ for all $x \in(a, b)$
Given:-
$f(x)=2 \log (x-2)-x^{2}+4 x+1$
$\mathrm{d}\left(\frac{\mathrm{f}(\mathrm{x})}{\mathrm{dx}}\right)=\frac{2}{\mathrm{x}-2}-2 \mathrm{x}+4=\mathrm{f}^{\prime}(\mathrm{x})$
$\Rightarrow f^{\prime}(x)=-\frac{2(x-1)(x-3)}{x-2}$
now for increasing
$f^{\prime}(x)>0$
$\Rightarrow-\frac{2(x-1)(x-3)}{x-2}<0$
$x-3<0$ and $x-2>0$
$x<3$ and $x>2$
$x \in(2,3)$