Why Doesn't Arccos(Cos x) Yield y = x?

In summary, the conversation discusses the question of why the equation y = arccos(cos x) does not yield y = x. The reason is due to the restrictions on the domain and range of trigonometric functions. The graph of y = cos(x) does not pass the horizontal line test, making it not an invertible function. This means that arccos x can only exist under certain restrictions. The domain of arccos x is [-1, 1] and the range is [0, pi]. Since the range of arccos(cos x) cannot be outside of [0, pi], and the domain of cos x is between -1 and 1, arccos(cos x) can be defined for
  • #1
NightSky72
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Homework Statement


The problem here involves inverse trig functions. The question here is why doesn't arccos(cos x) yield the equation y = x?


Homework Equations


y = arccos(cos x)


The Attempt at a Solution


I assume the reason is due to restrictions on domain and/or range of the trig functions. I graphed it out from -4pi to 4pi, and noticed that from 0 to pi, 2pi to 3pi, etc., it does display a function similar y = x (0 to pi it IS y = x). However, on intervals such as -pi to 0, it is the same as the function y = -x. Now, I looked at the unit circle and can see that, if it is the arccos (cos x), the cos of radian measurements (essentially the sine when reading on the unit circle) is strictly positive. However, between -pi and 0, the values would be negative. They increase and decrease graphically, respectively. Anyone have any input or guiding suggestions here? Thanks.
 
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  • #2
Start by looking at the graph of y=cos(x) and asking if it is an invertible function.
As such it is not! (Horizontal line test!)
What then does arccos mean?
 
  • #3
That means arccos x can only exist under certain restrictions, since an equation does not have an inverse unless its graph passes the horizontal line test. The domain of arccos x is [-1, 1] and the range is [0, pi]. Ok, so if the range can only be 0 to pi, then the graph of arccos (cos x) cannot have a range outside of that. And since the cos x only ranges between -1 and 1, then arccos (cos x) can be defined for any value of x, since the cos x is between -1 and 1, which is the domain of arccos x?
 

FAQ: Why Doesn't Arccos(Cos x) Yield y = x?

What are inverse trig functions?

Inverse trig functions are also known as arc trigonometric functions. They are the inverse functions of the basic trigonometric functions (sine, cosine, tangent, cotangent, secant, and cosecant). They are used to find the angle measure in a right triangle when given the ratio of two sides.

What is the notation used for inverse trig functions?

The notation used for inverse trig functions is "arc" followed by the abbreviation of the corresponding trigonometric function (e.g. arcsin for inverse sine, arccos for inverse cosine, arctan for inverse tangent, etc.). Another common notation is to use the exponent "-1" (e.g. sin⁻¹, cos⁻¹, tan⁻¹, etc.).

How do you solve an inverse trig function problem?

To solve an inverse trig function problem, you need to use the inverse trig function to find the angle measure. First, identify which trig function is given in the problem (sine, cosine, tangent, etc.). Then, use the inverse trig function of that function to solve for the angle measure. Finally, use a calculator or trigonometric tables to find the numerical value of the angle measure.

What is the domain and range of inverse trig functions?

The domain of inverse trig functions is the set of real numbers between -1 and 1, since the output of a trig function is always within this range. The range of inverse trig functions is the set of real numbers between -π/2 and π/2 for inverse sine and inverse tangent, and between 0 and π for inverse cosine and inverse secant.

How do inverse trig functions relate to right triangles?

Inverse trig functions are used to find the angle measures in a right triangle when given the ratio of two sides. This is because the trigonometric ratios (sine, cosine, tangent, etc.) are defined as the ratio of two sides in a right triangle. By using the inverse trig functions, we can "undo" the ratios and find the angle measures in a right triangle.

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