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Write the following functions in simplest form: $\tan^{-1}\left(\frac{\cos(x)-\sin(x)}{\cos(x)+\sin(x)}\right), \quad 0

Please help me to solve this problem. I have been trying to solve this from last 3 hours. I can solve simple inverse trigonometric functions

  • 0
    It would be nice if it had the form $\tan^{-1}\left(\frac{\sin(\cdots)}{\cos(\cdots)}\right)$, wouldn't it? See if you can use rewritings like [this](http://math.stackexchange.com/questions/181676/how-to-see-sin-x-cos-x/181841#181841) to make it so.2012-08-23

4 Answers 4

4

Let's multiply the numerator and denominator by $\ \cos\bigl(\frac {\pi}4\bigr)=\sin\bigl(\frac {\pi}4\bigr)\ $ : $\tan^{-1}\left(\frac{\sin\bigl(\frac {\pi}4\bigr)\cos(x)-\cos\bigl(\frac {\pi}4\bigr)\sin(x)}{\cos\bigl(\frac {\pi}4\bigr)\cos(x)+\sin\bigl(\frac {\pi}4\bigr)\sin(x)}\right),\quad 0

$=\tan^{-1}\left(\frac{\sin\left(\frac {\pi}4-x\right)}{\cos\left(\frac {\pi}4-x\right)}\right),\quad 0 $=\begin{cases} &\frac {\pi}4-x&\quad\text{if}\ \ 0 < x < \frac{3\pi}4\\ &\text{not defined}&\quad\text{if}\quad x = \frac{3\pi}4\\ &\frac {5\pi}4-x&\quad\text{if}\ \ \frac{3\pi}4< x < \pi\\ \end{cases} $

2

Let $\theta=\tan^{-1}(X)$ where $X=\left(\frac{\cos(x)-\sin(x)}{\cos(x)+\sin(x)}\right)$ and $0 ,we have $\frac{1-X}{1+X}=\tan(x)$ But $\tan(\theta)=X$ so, $\tan(x)=\frac{1-\tan(\theta)}{1+\tan(\theta)}=\tan(\frac{\pi}{4}-\theta)$. The rest is as Raymond concluded above.

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    Nice $\checkmark\quad\ddot\smile$2013-03-13
1

$\tan^{-1}\left(\frac{\cos(x)-\sin(x)}{\cos(x)+\sin(x)}\right),\quad 0

$=\tan^{-1}\left(\frac{1-\tan(x)}{1+\tan(x)}\right)$ diving the numerator and denominator by $\cos x$

$=\tan^{-1}\left(\frac{\tan\frac{\pi}{4}-\tan(x)}{1+\tan\frac{\pi}{4}\tan(x)}\right)$ as $\tan\frac{\pi}{4}=1$

$=\tan^{-1}\tan(\frac{\pi}{4}-x)$

$=n\pi+\frac{\pi}{4}-x$ where n is any integer.

The principal value which must lie in $[-\frac{\pi}{2},\frac{\pi}{2}]$, will be

$\frac{\pi}{4}-x$ when $\frac{\pi}{4}-x$ lies in that region i.e, $\frac{3\pi}{4} ≥ x > -\frac{\pi}{4}$

and $\pi +\frac{\pi}{4}-x$ elsewhere.

But $0,

so, if $\frac{\pi}{2} ≥ x ≥ 0$ the principal value =$\frac{\pi}{4}-x$

and $\frac{5\pi}{4}-x$ elsewhere.

-2

It is $\tan ^{-1}\dfrac{\cos \left(x\right)-\sin \left(x\right)}{\cos \left(x\right)+\sin \left(x\right)}$

We will divide in bracket with cosx to get it in tan form which will be easy for us to simplify

$\implies \tan ^{-1}\dfrac{\dfrac{\cos \left(x\right)-\sin \left(x\right)}{\cos \left(x\right)}}{\dfrac{\cos \left(x\right)-\sin \left(x\right)}{\cos \left(x\right)}}$

$\implies \tan ^{-1}\dfrac{1-\tan \left(x\right)}{1+\cos \left(x\right)}$

Now we know that $\tan ^{-1}x + \tan ^{-1}y= \dfrac{x-y}{1-xy}$ when $xy<1$

and we have the bracket in the same form as $\dfrac{\tan \left(1\right)-\tan \left(x\right)}{1-\tan \left(1\right)\tan \left(x\right)}$

So we get $\tan ^{-1} \left(\tan \left(\dfrac{π}{4}\right) + \tan \left(X\right)\right)$ i.e $\dfrac{π}{4-x}$.