Problem 7 - Single Variable Calculus

Question

Prove the identity. \(\cosh (-x)=\cosh x\)

Step-by-Step Solution

Answer

To prove the identity \(\cosh (-x) = \cosh x\), we first recall the definition of the hyperbolic cosine function, \(\cosh x = \frac{e^x + e^{-x}}{2}\). Then, we evaluate \(\cosh(-x)\) using the definition and obtain \(\cosh(-x) = \frac{e^{-x} + e^{x}}{2}\). By applying exponent properties and making substitutions, we rewrite the expression for \(\cosh(-x)\) and observe that it is the same as the original definition of \(\cosh x\). Therefore, we have proved the identity \(\cosh (-x) = \cosh x\).

Step 1: Definition of hyperbolic cosine function

Recall the definition of the hyperbolic cosine function, which is given by: \[\cosh x = \frac{e^x + e^{-x}}{2}\]

Step 2: Apply the definition to \(\cosh(-x)\)

Using the definition of the hyperbolic cosine function, evaluate \(\cosh(-x)\): \[\cosh(-x) = \frac{e^{-x} + e^{x}}{2}\]

Step 3: Apply the exponent properties

Note that \(e^{-x}\) can be rewritten as: \[\frac{1}{e^x}\] And \(e^x\) can be rewritten as: \[\frac{1}{e^{-x}}\]

Step 4: Substitute exponent properties

Substitute the rewritten exponent properties back into the expression for \(\cosh(-x)\): \[\cosh(-x) = \frac{\frac{1}{e^x} + \frac{1}{e^{-x}}}{2}\]

Step 5: Identify the original definition of \(\cosh x\)

We can observe that the expression for \(\cosh(-x)\) is the same as the original definition of \(\cosh x\): \[\cosh(-x) = \frac{e^x + e^{-x}}{2} = \cosh x\] So, we have proved the identity \(\cosh (-x) = \cosh x\).

Key Concepts

Hyperbolic Cosine

The hyperbolic cosine function, denoted as \( \cosh x \), is one of the basic hyperbolic functions. It is similar to the trigonometric cosine function but is based on exponential functions rather than circular functions. The formula to compute the hyperbolic cosine is defined as follows:

• \( \cosh x = \frac{e^x + e^{-x}}{2} \)

In this formula, \( e^x \) is the exponential function where \( e \) is the base of the natural logarithm, approximately equal to 2.71828. What's interesting about \( \cosh x \) is that it inherently describes the shape of a catenary curve, which is the curve formed by a hanging chain or cable.
Another key property of \( \cosh x \) is its evenness. This means that \( \cosh(-x) = \cosh x \); this specific property explains that the function behaves symmetrically on either side of the y-axis. It's similar to how a mirror image of a catenary will look about the y-axis.

Identity Proof

Proving identities in mathematics helps establish the truth of an equation universally. For the specific identity \( \cosh(-x) = \cosh x \), it is important to start with the definition of the hyperbolic cosine function and work through the steps rigorously. Here's a rundown of how the proof works:

• Start with the definition of \( \cosh x \), which is \( \frac{e^x + e^{-x}}{2} \).
• To find \( \cosh(-x) \), replace \( x \) with \( -x \) in the definition. This gives \( \frac{e^{-x} + e^x}{2} \).
• Notice that the expression \( e^{-x} + e^x \) is exactly the same as \( e^x + e^{-x} \), illustrating that \( \cosh(-x) = \cosh x \).

This identity emphasizes the even function property of hyperbolic cosine, where inputs positive or negative render the same result. Understanding this proof involves manipulating exponential expressions, which brings us to exploring the exponential functions themselves.

Exponential Functions

Exponential functions are mathematical expressions in which variables appear as exponents. One of the simplest forms of an exponential function is \( f(x) = a^x \), where "a" is a constant, and "x" is a variable. The most important base for these functions in advanced math and calculus is the number \( e \), leading to the expression \( e^x \).In the context of hyperbolic functions, exponential functions are used in defining both \( \sinh x\) and \( \cosh x \), where:

• \( \cosh x = \frac{e^x + e^{-x}}{2} \)
• \( \sinh x = \frac{e^x - e^{-x}}{2} \)

These provide a framework for not only hyperbolic trigonometry but also advanced topics like differential equations and complex analysis.
Exponential functions are pivotal due to their growth properties and their slopes that are proportional to the height at every point. They appear frequently in natural processes, such as compound interest, population growth, and radioactive decay. In hyperbolic functions, exponentiality is used to describe functions that simulate hyperbola-like structures. Understanding and manipulating exponential forms enable proofs, like that of the hyperbolic cosine identity, to be established with precision.