If $f(x)=\frac{x}{\sin x}$ and $g(x)=\frac{x}{\tan x}$, where $0<x \leq 1$, then in this interval |
both f(x) and g(x) are increasing functions both f(x) and g(x) are decreasing functions f(x) is an increasing function g(x) is an increasing function |
f(x) is an increasing function |
We have, $f(x)=\frac{x}{\sin x}$ and $g(x)=\frac{x}{\tan x}$ ∴ $f'(x)=\frac{\sin x-x \cos x}{\sin ^2 x}$ and $g'(x)=\frac{\tan x-x \sec ^2 x}{\tan ^2 x}$ Let $\phi(x)=\sin x-x \cos x$ and $\psi(x)=\tan x-x \sec ^2 x$ Then, $f(x)=\frac{\phi(x)}{\sin ^2 x}$ and $g'(x)=\frac{\psi(x)}{\tan ^2 x}$ Now, $\phi'(x)=\cos x-\cos x+x \sin x=x \sin x$ and, $\psi'(x)=\sec ^2 x-\sec ^2 x-2 x \sec ^2 x \tan x=-2 x \sec ^2 x \tan x$ For $0<x \leq 1$, we have $x>0, \sin x>0, \tan x>0, \sec x>0$ $\Rightarrow \phi'(x)=x \sin x>0$ and $\psi'(x)<0$ for $0<x \leq 1$ $\Rightarrow \phi(x)$ is increasing on (0,1] and $\psi(x)$ is decreasing on (0,1] $\Rightarrow \phi(x)>\phi(0)$ and $\psi(x)<\psi(0)$ $\Rightarrow \phi(x)>0$ and $\psi(x)<0$ $\Rightarrow f'(x)=\frac{\phi(x)}{\sin ^2 x}>0$ and $g'(x)=\frac{\phi(x)}{\tan ^2 x}<0$ $\Rightarrow \quad f(x)$ is increasing on $(0,1]$ and $g(x)$ is decreasing on $(0,1]$. |