Problem 15 - Calculus Early Transcendentals

Question

Verify each identity using the definitions of the hyperbolic functions. \(\cosh x+\sinh x=e^{x}\)

Step-by-Step Solution

Answer

Question: Verify the identity involving hyperbolic functions: \(\cosh x + \sinh x = e^x\). Answer: Using the definitions of hyperbolic functions, we rewrote the identity and simplified the equation to show that both sides are equal, confirming the identity \(\cosh x + \sinh x = e^x\).

Step 1: Recall the definitions of hyperbolic functions

The hyperbolic cosine and hyperbolic sine functions are defined as follows: $$\cosh x = \frac{e^x + e^{-x}}{2}$$ $$\sinh x = \frac{e^x - e^{-x}}{2}$$ We will use these definitions to rewrite the identity we want to verify.

Step 2: Rewrite the identity using the definitions

We replace \(\cosh x\) and \(\sinh x\) in the identity with their definitions from Step 1: $$\frac{e^x + e^{-x}}{2} + \frac{e^x - e^{-x}}{2} = e^x$$

Step 3: Simplify the left side of the equation

We have two fractions with the same denominator, so we can combine them: $$\frac{e^x + e^{-x} + e^x - e^{-x}}{2} = e^x$$ Now, we can simplify the numerator by combining like terms: $$\frac{2e^x}{2} = e^x$$

Step 4: Further simplify and confirm the identity

We can now cancel the 2 in the numerator and denominator, leaving: $$e^x = e^x$$ Since both sides of the equation are equal, we've confirmed the identity \(\cosh x + \sinh x = e^x\).

Key Concepts

Hyperbolic Cosine

The hyperbolic cosine function, denoted as \(\cosh x\), is an important mathematical function that falls under the category of hyperbolic functions. It is defined using the exponential function, which might seem intimidating, but in reality, it's quite straightforward. The formula for hyperbolic cosine is: \[ \cosh x = \frac{e^x + e^{-x}}{2} \] This expression indicates that \(\cosh x\) is the average of \(e^x\) and \(e^{-x}\).

• \(e^x\) is the exponential function, which describes exponential growth.
• \(e^{-x}\) is the exponential decay represented by the inverse exponential function.

The hyperbolic cosine function is similar in terms of properties to the cosine function we know from trigonometry, but it relates to the geometry of hyperbolas rather than circles. It's even in nature, meaning \(\cosh(-x) = \cosh(x)\), reflecting along the y-axis.
In practical terms, \(\cosh x\) finds applications in engineering and physics, particularly in calculating catenary curves related to hanging cables.

Hyperbolic Sine

Hyperbolic sine, denoted as \(\sinh x\), is another fundamental hyperbolic function which complements the hyperbolic cosine function. It has a distinct definition and properties: \[ \sinh x = \frac{e^x - e^{-x}}{2} \] Here, \(\sinh x\) is calculated by taking the difference between \(e^x\) and \(e^{-x}\), then dividing by 2.

• \(e^x\): This parts adds more weight to the positive growth direction.
• \(e^{-x}\): Subtracting this component adjusts for the decay direction.

Unlike \(\cosh x\), the hyperbolic sine function is odd, meaning it satisfies the relation \(\sinh(-x) = -\sinh(x)\). This characteristic has practical implications such as in solving differential equations and modeling hyperbolic geometry.
By nature, \(\sinh x\) offers insights into behaviors that are seen in rapid growth and then decline, akin to natural processes in growth rate and physics.

Mathematical Identity Verification

The process of mathematical identity verification is a way to confirm that two expressions are, in fact, the same, achieved by simplifying or manipulating them. In the context of hyperbolic functions, verifying identities like \(\cosh x + \sinh x = e^x\) is essential.
These identities can be verified by substituting the definitions of \(\cosh x\) and \(\sinh x\) as given in previous sections:

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

When substituting these values in \(\cosh x + \sinh x\), you have:\[ \frac{e^x + e^{-x}}{2} + \frac{e^x - e^{-x}}{2} = e^x\] Combining like terms gives \(\frac{2e^x}{2} = e^x\), confirming the identity as the equation simplifies to the same expression on both sides.
Verification of identities helps in understanding and trusting complex mathematical constructs, allowing for clear solutions in varied applications of science and engineering.