Inverse Trigonometric Functions

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, as summarized in [link].

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:

• Since  sin( π 6 )= 1 2 ,  then   π 6 = sin −1 ( 1 2 ).
• Since  cos( π )=−1,  then  π= cos −1 ( −1 ).
• Since  tan( π 4 )=1,  then   π 4 = tan −1 ( 1 ).

In previous sections, we evaluated the trigonometric functions at various angles, but at times we need to know what angle would yield a specific sine, cosine, or tangent value. For this, we need inverse functions. Recall that, for a one-to-one function, if  f(a)=b,  then an inverse function would satisfy   f −1 (b)=a.

Bear in mind that the sine, cosine, and tangent functions are not one-to-one functions. The graph of each function would fail the horizontal line test. In fact, no periodic function can be one-to-one because each output in its range corresponds to at least one input in every period, and there are an infinite number of periods. As with other functions that are not one-to-one, we will need to restrict the domain of each function to yield a new function that is one-to-one. We choose a domain for each function that includes the number 0. [link] shows the graph of the sine function limited to  [ − π 2 , π 2 ]  and the graph of the cosine function limited to  [ 0,π ].

[link] shows the graph of the tangent function limited to  ( − π 2 , π 2 ).

These conventional choices for the restricted domain are somewhat arbitrary, but they have important, helpful characteristics. Each domain includes the origin and some positive values, and most importantly, each results in a one-to-one function that is invertible. The conventional choice for the restricted domain of the tangent function also has the useful property that it extends from one vertical asymptote to the next instead of being divided into two parts by an asymptote.

On these restricted domains, we can define the inverse trigonometric functions.

• The inverse sine function  y= sin −1 x  means  x=sin y.  The inverse sine function is sometimes called the arcsine function, and notated  arcsinx.
  y= sin −1 x has domain [−1,1] and range [ − π 2 , π 2 ]
• The inverse cosine function  y= cos −1 x  means  x=cos y.  The inverse cosine function is sometimes called the arccosine function, and notated  arccos x.
  y= cos −1 x has domain [−1,1] and range [0,π]
• The inverse tangent function  y= tan −1 x  means  x=tan y.  The inverse tangent function is sometimes called the arctangent function, and notated  arctan x.
  y= tan −1 x has domain (−∞,∞) and range ( − π 2 , π 2 )

The graphs of the inverse functions are shown in [link], [link], and [link]. Notice that the output of each of these inverse functions is a number, an angle in radian measure. We see that   sin −1 x  has domain  [ −1,1 ]  and range  [ − π 2 , π 2 ], cos −1 x  has domain  [ −1,1 ]  and range  [0,π],  and   tan −1 x  has domain of all real numbers and range  ( − π 2 , π 2 ).  To find the domain and range of inverse trigonometric functions, switch the domain and range of the original functions. Each graph of the inverse trigonometric function is a reflection of the graph of the original function about the line  y=x.

Now that we can identify inverse functions, we will learn to evaluate them. For most values in their domains, we must evaluate the inverse trigonometric functions by using a calculator, interpolating from a table, or using some other numerical technique. Just as we did with the original trigonometric functions, we can give exact values for the inverse functions when we are using the special angles, specifically   π 6   (30°),   π 4   (45°), and   π 3   (60°), and their reflections into other quadrants.

To evaluate inverse trigonometric functions that do not involve the special angles discussed previously, we will need to use a calculator or other type of technology. Most scientific calculators and calculator-emulating applications have specific keys or buttons for the inverse sine, cosine, and tangent functions. These may be labeled, for example, SIN-1, ARCSIN, or ASIN.

In the previous chapter, we worked with trigonometry on a right triangle to solve for the sides of a triangle given one side and an additional angle. Using the inverse trigonometric functions, we can solve for the angles of a right triangle given two sides, and we can use a calculator to find the values to several decimal places.

In these examples and exercises, the answers will be interpreted as angles and we will use  θ  as the independent variable. The value displayed on the calculator may be in degrees or radians, so be sure to set the mode appropriate to the application.

How To

Given two sides of a right triangle like the one shown in [link], find an angle.

1. If one given side is the hypotenuse of length  h  and the side of length  a  adjacent to the desired angle is given, use the equation   θ= cos −1 ( a h ).
2. If one given side is the hypotenuse of length  h  and the side of length  p  opposite to the desired angle is given, use the equation  θ= sin −1 ( p h ).
3. If the two legs (the sides adjacent to the right angle) are given, then use the equation  θ= tan −1 ( p a ).

Applying the Inverse Cosine to a Right Triangle

Solve the triangle in [link] for the angle  θ.

Because we know the hypotenuse and the side adjacent to the angle, it makes sense for us to use the cosine function.

cos θ= 9 12          θ= cos −1 ( 9 12 ) Apply definition of the inverse.          θ≈0.7227 or about 41.4096° Evaluate.

Try It

Solve the triangle in [link] for the angle  θ.

sin −1 (0.6)=36.87°=0.6435  radians

There are times when we need to compose a trigonometric function with an inverse trigonometric function. In these cases, we can usually find exact values for the resulting expressions without resorting to a calculator. Even when the input to the composite function is a variable or an expression, we can often find an expression for the output. To help sort out different cases, let  f(x)  and  g(x)  be two different trigonometric functions belonging to the set  { sin(x),cos(x),tan(x) }  and let   f −1 (y)  and   g −1 (y) be their inverses.

Evaluating Compositions of the Form f⁻¹(g(x))

Now that we can compose a trigonometric function with its inverse, we can explore how to evaluate a composition of a trigonometric function and the inverse of another trigonometric function. We will begin with compositions of the form   f −1 ( g( x ) ).  For special values of  x, we can exactly evaluate the inner function and then the outer, inverse function. However, we can find a more general approach by considering the relation between the two acute angles of a right triangle where one is  θ,  making the other   π 2 −θ. Consider the sine and cosine of each angle of the right triangle in [link].

Right triangle illustrating the cofunction relationships

Because  cos θ= b c =sin( π 2 −θ ),  we have   sin −1 ( cos θ )= π 2 −θ  if  0≤θ≤π.  If  θ  is not in this domain, then we need to find another angle that has the same cosine as  θ  and does belong to the restricted domain; we then subtract this angle from   π 2 . Similarly,  sin θ= a c =cos( π 2 −θ ),  so   cos −1 ( sin θ )= π 2 −θ  if  − π 2 ≤θ≤ π 2 .  These are just the function-cofunction relationships presented in another way.

How To

Given functions of the form   sin −1 ( cos x )  and   cos −1 ( sin x ),  evaluate them.

1. If  x is in [ 0,π ],  then   sin −1 ( cos x )= π 2 −x.
2. If  x is not in [ 0,π ],  then find another angle  y in [ 0,π ]  such that  cos y=cos x.
   sin −1 ( cos x )= π 2 −y
3. If  x is in [ − π 2 , π 2 ],  then   cos −1 ( sin x )= π 2 −x.
4. If  x is not in [ − π 2 , π 2 ],  then find another angle  y in [ − π 2 , π 2 ]  such that  sin y=sin x.
   cos −1 ( sin x )= π 2 −y

Evaluating the Composition of an Inverse Sine with a Cosine

Evaluate   sin −1 ( cos( 13π 6 ) )

1. by direct evaluation.
2. by the method described previously.

1. Here, we can directly evaluate the inside of the composition.
   cos( 13π 6 )=cos( π 6 +2π)                =cos( π 6 )                = 3 2

   Now, we can evaluate the inverse function as we did earlier.

   sin −1 ( 3 2 )= π 3
2. We have  x= 13π 6 , y= π 6 ,  and
   sin −1 ( cos( 13π 6 ) )= π 2 − π 6 = π 3        

Try It

Evaluate   cos −1 ( sin( − 11π 4 ) ).

3π 4

Evaluating Compositions of the Form f(g⁻¹(x))

To evaluate compositions of the form  f( g −1 ( x ) ),  where  f  and  g  are any two of the functions sine, cosine, or tangent and  x  is any input in the domain of   g −1 ,  we have exact formulas, such as  sin( cos −1 x )= 1− x 2 .  When we need to use them, we can derive these formulas by using the trigonometric relations between the angles and sides of a right triangle, together with the use of Pythagoras’s relation between the lengths of the sides. We can use the Pythagorean identity,   sin 2 x+ cos 2 x=1,  to solve for one when given the other. We can also use the inverse trigonometric functions to find compositions involving algebraic expressions.

Evaluating the Composition of a Sine with an Inverse Cosine

Find an exact value for  sin( cos −1 ( 4 5 ) ).

Beginning with the inside, we can say there is some angle such that  θ= cos −1 ( 4 5 ),  which means  cos θ= 4 5 ,  and we are looking for  sin θ.  We can use the Pythagorean identity to do this.

sin 2 θ+ cos 2 θ=1 Use our known value for cosine.     sin 2 θ+ ( 4 5 ) 2 =1 Solve for sine.                       sin 2 θ=1− 16 25                         sin θ=± 9 25 =± 3 5

Since  θ= cos −1 ( 4 5 )  is in quadrant I,  sin θ  must be positive, so the solution is   3 5 .  See [link].

Right triangle illustrating that if  cos θ= 4 5 ,  then  sin θ= 3 5  

We know that the inverse cosine always gives an angle on the interval  [ 0,π ],  so we know that the sine of that angle must be positive; therefore  sin( cos −1 ( 4 5 ) )=sin θ= 3 5 .

Try It

Evaluate  cos( tan −1 ( 5 12 ) ).

12 13

Evaluating the Composition of a Sine with an Inverse Tangent

Find an exact value for  sin( tan −1 ( 7 4 ) ).

While we could use a similar technique as in [link], we will demonstrate a different technique here. From the inside, we know there is an angle such that  tan θ= 7 4 .  We can envision this as the opposite and adjacent sides on a right triangle, as shown in [link].

A right triangle with two sides known

Using the Pythagorean Theorem, we can find the hypotenuse of this triangle.

       4 2 + 7 2 = hypotenuse 2 hypotenuse= 65

Now, we can evaluate the sine of the angle as the opposite side divided by the hypotenuse.

sin θ= 7 65

This gives us our desired composition.

sin( tan −1 ( 7 4 ) )=sin θ                       = 7 65                       = 7 65 65

Try It

Evaluate  cos( sin −1 ( 7 9 ) ).

4 2 9

Finding the Cosine of the Inverse Sine of an Algebraic Expression

Find a simplified expression for  cos( sin −1 ( x 3 ) )  for  −3≤x≤3.

We know there is an angle  θ  such that  sin θ= x 3 .

sin 2 θ+ cos 2 θ=1 Use the Pythagorean Theorem.   ( x 3 ) 2 + cos 2 θ=1 Solve for cosine.               cos 2 θ=1− x 2 9                 cosθ=± 9− x 2 9 =± 9− x 2 3

Because we know that the inverse sine must give an angle on the interval  [ − π 2 , π 2 ],  we can deduce that the cosine of that angle must be positive.

cos( sin −1 ( x 3 ) )= 9− x 2 3

Try It

Find a simplified expression for  sin( tan −1 ( 4x ) )  for  − 1 4 ≤x≤ 1 4 .

4x 16 x 2 +1

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