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Question: Which of the following corresponds to the principal value branch of \({{\tan }^{-1}} {x}\)? A. \(\...

Which of the following corresponds to the principal value branch of tan1x{{\tan }^{-1}} {x}?
A. (π2,π2)\left( -\dfrac{\pi }{2},\dfrac{\pi }{2} \right)
B. [π2,π2]\left[ -\dfrac{\pi }{2},\dfrac{\pi }{2} \right]
C. \left( -\dfrac{\pi }{2},\dfrac{\pi }{2} \right) - \left\\{ 0 \right\\}
D. (0,π)\left( 0,\pi \right)

Explanation

Solution

We explain the function tan1{{\tan }^{-1}}. We express the inverse function of tan in the form of arctan(x)=tan1xarc\tan \left( x \right)={{\tan }^{-1}}x. We draw the graph of tan1(x){{\tan }^{-1}}\left( x \right) and take an example of tan11{{\tan }^{-1}}1 to understand the exact and general solution.

Complete step by step answer:
The given expression is the inverse function of trigonometric ratio tan. The arcus function represents the angle which on ratio tan gives the value.So, arctan(x)=tan1xarc\tan \left( x \right)={{\tan }^{-1}}x. If arctan(x)=αarc\tan \left( x \right)=\alpha then we can say tanα=x\tan \alpha =x.Each of the trigonometric functions is periodic in the real part of its argument, running through all its values twice in each interval of 2π2\pi . The general solution for that value where tanα=x\tan \alpha =x will be nπ+α,nZn\pi +\alpha ,n\in \mathbb{Z}.

But for arctan(x)arc\tan \left( x \right), we won’t find the general solution. We use the principal value. For ratio tan we have π2<arctan(x)<π2-\dfrac{\pi }{2} < arc\tan \left( x \right) < \dfrac{\pi }{2}.The graph of the function is,

arctan(x)=αarc\tan \left( x \right)=\alpha gives the angle α\alpha behind the ratio. We now take the example of x=1x=1 in the function of arctan(x)arc\tan \left( x \right). Let the angle be θ\theta for which arctan(1)=θarc\tan \left( 1 \right)=\theta . This gives tanθ=1\tan \theta =1. Putting the value in the graph of arctan(x)arc\tan \left( x \right), we get θ=45\theta =45.

For this we take the line of x=1x=1 and see the intersection of the line with the graph arctan(x)arc\tan \left( x \right).

The correct option is A.

Note: Although for elementary knowledge the principal domain is enough to solve the problem. But if mentioned to find the general solution then the domain changes to π2<arctan(x)<π2-\dfrac{\pi }{2} < arc\tan \left( x \right) < \dfrac{\pi }{2}. In that case we have to use the formula x=nπ+ax=n\pi +a for tan(x)=tana\tan \left( x \right)=\tan a where π2<a<π2-\dfrac{\pi }{2} < a < \dfrac{\pi }{2}.