Question
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 tan−1x?
A. (−2π,2π)
B. [−2π,2π]
C. \left( -\dfrac{\pi }{2},\dfrac{\pi }{2} \right) - \left\\{ 0 \right\\}
D. (0,π)
Solution
We explain the function tan−1. We express the inverse function of tan in the form of arctan(x)=tan−1x. We draw the graph of tan−1(x) and take an example of tan−11 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)=tan−1x. If arctan(x)=α then we can say tanα=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 2π. The general solution for that value where tanα=x will be nπ+α,n∈Z.
But for arctan(x), we won’t find the general solution. We use the principal value. For ratio tan we have −2π<arctan(x)<2π.The graph of the function is,
arctan(x)=α gives the angle α behind the ratio. We now take the example of x=1 in the function of arctan(x). Let the angle be θ for which arctan(1)=θ. This gives tanθ=1. Putting the value in the graph of arctan(x), we get θ=45.
For this we take the line of x=1 and see the intersection of the line with the graph arctan(x).
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 −2π<arctan(x)<2π. In that case we have to use the formula x=nπ+a for tan(x)=tana where −2π<a<2π.