Problem 12 - Thomas Calculus

Question

Use the definitions of cosh x and sinh x to show that $$\cosh ^{2} x-\sinh ^{2} x=1$$

Step-by-Step Solution

Answer

The identity \( \cosh^2 x - \sinh^2 x = 1 \) is proven using the definitions of \( \cosh x \) and \( \sinh x \).

Step 1: Recall Definitions

Recall the definitions of hyperbolic cosine and hyperbolic sine: \( \cosh x = \frac{e^x + e^{-x}}{2} \) and \( \sinh x = \frac{e^x - e^{-x}}{2} \). We will use these definitions in the next steps.

Step 2: Square Each Function

Calculate \( \cosh^2 x \) and \( \sinh^2 x \): \( \cosh^2 x = \left( \frac{e^x + e^{-x}}{2} \right)^2 = \frac{(e^x + e^{-x})^2}{4} \) and \( \sinh^2 x = \left( \frac{e^x - e^{-x}}{2} \right)^2 = \frac{(e^x - e^{-x})^2}{4} \).

Step 3: Expand Squares

Expand \((e^x + e^{-x})^2\) and \((e^x - e^{-x})^2\): \( (e^x + e^{-x})^2 = e^{2x} + 2 + e^{-2x} \) and \( (e^x - e^{-x})^2 = e^{2x} - 2 + e^{-2x} \).

Step 4: Substitute Results

Substitute the results from Step 3 back into the expressions for \( \cosh^2 x \) and \( \sinh^2 x \): \( \cosh^2 x = \frac{e^{2x} + 2 + e^{-2x}}{4} \) and \( \sinh^2 x = \frac{e^{2x} - 2 + e^{-2x}}{4} \).

Step 5: Calculate \( \cosh^2 x - \sinh^2 x \)

Subtract \( \sinh^2 x \) from \( \cosh^2 x \): \[ \cosh^2 x - \sinh^2 x = \frac{(e^{2x} + 2 + e^{-2x}) - (e^{2x} - 2 + e^{-2x})}{4} = \frac{4}{4} = 1 \].

Step 6: Conclude the Identity

Thus, by simplifying the expression, we have shown that \( \cosh^2 x - \sinh^2 x = 1 \). This confirms the identity.

Key Concepts

cosh (hyperbolic cosine)

The hyperbolic cosine function, represented as \( \cosh x \), is a fundamental building block of hyperbolic functions. It's similar to the cosine function in trigonometry but adapted for hyperbolic geometry. The formula for \( \cosh x \) is given by:

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

Here, \( e \) is the base of the natural logarithm, approximately 2.718. The hyperbolic cosine function is even due to its symmetry around the y-axis. This means \( \cosh(-x) = \cosh(x) \).
\( \cosh x \) describes the shape of a hanging cable or chain, known as a catenary. In its properties, it shows exponential growth as \( |x| \) increases.
In terms of application, \( \cosh x \) frequently occurs in calculations involving the angles or slopes of hyperbolic relationships in real-world physics and engineering problems.

sinh (hyperbolic sine)

The hyperbolic sine function, denoted by \( \sinh x \), pairs with the hyperbolic cosine to describe hyperbolic functions. It mirrors the sine function from trigonometry but for hyperbolic spaces.

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

The key aspect of \( \sinh x \) is that it is an odd function: \( \sinh(-x) = -\sinh(x) \). This means the graph is symmetric about the origin, making it essential for representing vertical hyperbolic motion.
In practice, \( \sinh x \) is used to model phenomena such as the shape of a flexible beam subject to linear distributed loads.
Unlike the hyperbolic cosine, which has a minimum value of one, \( \sinh x \) can take any real number value, reflecting both positive and negative growth as \( x \) changes. It grows exponentially like \( \cosh x \) but without a horizontal baseline shift.

hyperbolic identities

Hyperbolic identities are various equations that involve hyperbolic functions, much like trigonometric identities involve sine and cosine. One central identity is:

• \( \cosh^2 x - \sinh^2 x = 1 \)

This identity resembles the Pythagorean identity \( \cos^2 x + \sin^2 x = 1 \), linking hyperbolic and circular functions.
The proof of this hyperbolic identity leverages the definitions of \( \cosh x \) and \( \sinh x \):

• Square \( \cosh x \) and \( \sinh x \)
• Plug in: \( \cosh^2 x = \frac{(e^x + e^{-x})^2}{4} \) and \( \sinh^2 x = \frac{(e^x - e^{-x})^2}{4} \)
• Simplify to show the identity

This relationship explains various natural phenomena, showing a simplified version of the exponential components of hyperbolic functions. It's used in physics, engineering, and complex analysis, where hyperbolic functions model wave behaviors and relativity scenarios. These hyperbolic identities help to solve real-life hyperbolic problems by simplifying complex exponential expressions.