Problem 45 - Precalculus : Building Concepts and Connections

Question

Explain why each of the given expressions is undefined. $$\arccos 4$$

Step-by-Step Solution

Answer

\(\arccos 4\) is undefined because 4 is not in the range of the arccosine function which is \([-1, 1]\)

Step 1: Understanding the range of \(\arccos(x)\)

First, you need to understand that the range of the function \(\arccos(x)\) is \([-1,1]\). This means that for any \(x\) outside of this range, \(\arccos(x)\) is undefined.

Step 2: Applying the knowledge to the given expression

In the given expression, we are asked to find the value of \( \arccos 4 \). Since 4 is outside of the range \([-1,1]\), hence \( \arccos 4 \) is undefined.

Key Concepts

Domain and Range

In the world of functions, the domain and range are two key concepts. The **domain** of a function refers to all the possible input values (or "x-values") that can be plugged into the function. Meanwhile, the **range** refers to all the possible output values (or "y-values") that the function can produce.

For inverse trigonometric functions like the arccosine function, understanding the domain and range is crucial. For the standard cosine function, which outputs values between -1 and 1, the inverse function, arccos(x), will only accept inputs (or x-values) in this same range. Therefore, the domain of the arccosine function is the interval [-1,1]. Meanwhile, the output values or range of \( ext{arccos}(x)\) is between 0 and \( ext{π}\) radians.

This means that arccos(x) will provide real-number outputs only for inputs between -1 and 1. Understanding this limitation helps us determine when an expression such as \( ext{arccos}(4)\) is undefined.

Arccos Function

The **arccos function** is a specific type of inverse trigonometric function. It essentially "undoes" what the cosine function does. While the cosine function takes an angle and returns its cosine value, the arccos function takes a cosine value and returns an angle.

The notation \( ext{arccos}(x)\) asks for "which angle's cosine equals x?" Since the \( ext{cosine}\) function's range is from -1 to 1, only these x values are valid inputs for the function. If you use a value outside this range for \(x\), like the number 4, it's impossible to find an angle whose cosine is this number. Hence, the expression becomes undefined.

You can think of the arccos function as defining a special and limited angle in the range 0 to \( ext{π}\) radians that corresponds to any valid input. This limited range for outputs guarantees that every input in the domain has exactly one output, making the arccos function a proper function.

Undefined Expressions

An expression becomes undefined when it involves operations or values that break the mathematical rules. For inverse trigonometric functions, undefined expressions often come from input values that don't lie within the necessary domain.

Using the example of \( ext{arccos}(4)\), the input value "4" exceeds the permitted domain of [-1,1]. Since the arccos function is defined only for x-values in this interval, trying to evaluate \( ext{arccos}(4)\) leads to an undefined expression.

Not only is the domain vital, but understanding the reasoning behind it can prevent mathematical errors. Always ensure that the input value for inverse trigonometric functions lies within the acceptable range so that the function can yield a valid output. Regularly checking if an expression is undefined can save time and frustration in solving problems.