Inverse Trigonometric Functions - Practice Questions & MCQ

Inverse Trigonometric Function

In order to use inverse trigonometric functions, we need to understand that an inverse trigonometric function “undoes” what the original trigonometric function “does,” as is the case with any other function and its inverse. In other words, the domain of the inverse function is the range of the original function and vice versa.

For example, if f(x) = sin x, then we would write f⁻¹(x) = sin⁻¹ x. Be aware that sin⁻¹ x does not mean 1/sinx. The following examples illustrate the inverse trigonometric functions:

1. sin (π/6) = ½, then π/6 = sin^-1 (½ ) 

2. cos(π) = −1, then π = cos⁻¹ (−1) 

3. tan (π/4)  = 1, then (π/4) = tan⁻¹ (1)

We know that inverse of a function is defined when the function is one-one and onto. But we also know that trigonometric functions are periodic and hence many-one in their domain.

So, to make inverse of trigonometric functions to be defined, the actual domain of trigonometric function must be restricted to make it a one-one function and its co-domain should be be restricted to make it an onto function.

The domain of the sine function is R and range is [-1, 1]. If we restrict its domain to [-π/2, π/2] then it become one-one and if we restrict its co-domain to [-1,1], then it becomes onto. So for this new domain and range, the inverse of our function y = sin(x) is defined.

And domain for this inverse function y = sin^-1(x) will be range of y = sin(x): [-1,1] and its range will be equal to domain of y= sin(x): 

[-π/2, π/2]

Actually, sine function can be restricted to any of the intervals [-3π/2, -π/2], [-π/2, π/2], [π/2, 3π/2] and so on. It becomes one-one in all these intervals and it is also onto (if codomain is [-1,1]). We can, therefore, define the inverse of sine function in each of these intervals. But by convention we take domain as [-π/2, π/2], and this domain is called Principal domain/ Principal value branch of y = sin(x)

So if f: [-π/2, π/2] → [-1,1]  and f(x) = sin(x), then its inverse is

f^-1:  [-1,1] → [-π/2, π/2] and f^-1(x) = sin^-1(x)

In a similar way, we define other trigonometric functions by restricting their domain and co-domains.