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MATHEMATICS - THEORY


No Inverse Function for
f(x) = sin x from -∞ to +∞
 

In this section, the method of finding the derivative of an inverse trigonometric function is introduced.

It is known that trigonometric functions are not one-to-one function, and thus they do not have inverse function. However, trigonometric functions can be converted to one-to-one functions by restricting their domains. For example, function f(x) = sin x is not a one-to-one function from -∞ to +∞ and has no inverse function. This can be seen on the left figure. However, it is a one-to-one function from -π/2 to +π/2, and thus f(x) has inverse function in this range.

     
    Arcsine Function


sin x and Its Inverse Function asin(x)
from -π/2 to +π/2
 

The inverse function of f(x) = sin x is called the arcsine function. According to the definition of the inverse function, the following two equations are obtained and they are reciprocal inverse functions.

     sin-1x = y

     sin y = x         for -π/2 ≤ y ≤ +π/2

The cancellation equation y = f(f -1(y)) and x = f -1(f(x)) for arcsine function becomes

     sin-1 (sin x) = x          for -π/2 ≤ x ≤ +π/2

     sin (sin-1 x) = x          for -1≤ x ≤ +1

The derivative of an arcsine function can be calculated as

     

where -1< x < +1

     
    Inverse Cosine Function

   

The inverse cosine function is denoted by cos-1 or arccos. Its definition states:

     cos-1x = y

     cos y = x      

are reciprocal inverse functions where 0 ≤ y ≤ π.

The cancellation equations for inverse cosine function are

     cos-1 (cos x) = x          for 0 ≤ x ≤ π

     cos (cos-1 x) = x          for -1 ≤ x ≤ +1

The derivative of the inverse cosine function is given by

     

     
    Inverse Tangent Function


tan x and Its Inverse Function atan x
in -π/2 to +π/2
 

The inverse tangent function is defined as the inverse function of tangent function and denoted as tan-1 or atan. Its domain is the interval (-π/2, +π/2). Its definition states:

     tan-1x = y

     tan y = x       

are reciprocal inverse functions where -π/2 ≤ y ≤ +π/2.

The arctangent function is horizontal asymptotes because when x approaches +∞, 1/x is extremely close to +π/2 and when x approaches -∞, 1/x is extremely close to -π/2. This can be expressed with mathematics equations:

     

and

     

The derivative of the inverse arctangent function is given by

     

     
  Other Inverse Trigonometric Function

Function Equation
 

The definition of csc-1 is given by

     sec-1x = y         for |x| ≥ 1

     sec y = x         for y ∈[0, π/2] ∪ [π/2, 0]

The definition of csc-1 states

     csc-1x = y         for |x| ≥ 1

     csc y = x         for y ∈(0, π/2] ∪ [π, 3π/2]

The definition of cot-1states

     cot-1x =& y     

     cot y = x         for y ∈(0, π)

   
  Derivative for Inverse Trigonometric Functions

 

Inverse trigonometric functions also have integration and derivation. Some of the formulas are listed in the table.