Trigonometric Functions - Domain and Range

Domain and Range of trigonometric functions

All trigonometric functions are basically the trigonometric ratios of any given angle. For example if we take the functions, f(x)=sin x, f(z) = tan z , etc, we are considering these trigonometric ratios as functions. Since they are considered to be functions, they will have some domain and range. In the upcoming discussion, we shall figure out the domain and range of trigonometric functions. To begin with, let us consider the simplest trigonometric identity:

sin2x + cos2 x = 1

From the given identity, following things can be interpreted:

cos2x = 1- sin2 x

cos x = √(1- sin2x)

Now we know that cosine function is defined for real values therefore the value inside the root is always non-negative. Therefore,

1- sin2x ≥ 0

sin2x ≤ 1

sin x ∈ [-1, 1]

Similarly, following the same methodology,

1- cos2x ≥ 0

cos2x ≤1

cos x ∈ [-1,1]

Hence for the trigonometric functions f(x)= sin x and f(x)= cos x, the domain will consist of the entire set of real numbers, as they are defined for all the real numbers. The range of f(x) = sin x and f(x)= cos x will lie from -1 to 1, including both -1 and +1, i.e.

-1 ≤sin x ≤1

-1 ≤cos x ≤1

Now, let us discuss about the function f(x)= tan x. We know, tan x = sin x / cos x. It means that tan x will be defined for all values except the values that will make cos x = 0, because a fraction with denominator 0 is not defined. Now we know that cos x is zero for the angles  π/2, 3 π/2, 5 π/2 etc therefore,

Trigonometric Functions

Hence for these values tan x is not defined.

So, the domain of f(x) = tan x  will be R – \(\frac{(2n+1)π}{2}\)  and the range will be set of all real numbers, R. We know that sec x, cosec x and cot x are the reciprocal of cos x, sin x and tan x respectively. Thus,

sec x = 1/cosx

cosec x = 1/sinx

cot x = 1/tanx

Hence, these ratios will not be defined for the following:

  1. sec x will not be defined at the points where cos x is 0. Hence the domain of sec x will be R-(2n+1)π/2, where n∈I. The range of sec x will be R- (-1,1). Since, cos x lies between -1 to1, so sec x can never lie between that region.
  2. cosec x will not be defined at the points where sin x is 0. Hence the domain of cosec x will be R-nπ, where n∈I. The range of cosec x will be R- (-1,1). Since, sin x lies between -1 to1, so cosec x can never lie in the region of -1 and 1.
  3. cot x will not be defined at the points where tan x is 0. Hence the domain of cot x will be R-nπ, where n∈I. The range of cot x will be the set of all real numbers, R.

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Practise This Question

In any ΔABC, the least value of (sin2A+sin A+1sin A) is