Problem 9 - Essential Calculus

Question

Verify the identity. \(\sinh (x+y)=\sinh x \cosh y+\cosh x \sinh y\)

Step-by-Step Solution

Answer

By expanding \( \sinh (x+y) \) using the definitions of hyperbolic functions, it can be shown that \( \sinh (x+y) = \sinh x \cosh y + \cosh x \sinh y \).

Step 1: Recognize The Hyperbolic Functions

The identity given contains hyperbolic functions. Recognize them and come to realize the task is to prove the equality of the two sides. This can be done by expanding \( \sinh (x+y) \) on the left and see if it equals the expression on the right.

Step 2: Expand Using The Definition

Expand the left side using the definition of hyperbolic functions. The definitions are: \( \sinh x = \frac{e^{x} - e^{-x}}{2} \) and \( \cosh x = \frac{e^{x} + e^{-x}}{2}\). Use these definitions to expand \( \sinh (x+y) \) to \( \sinh x \cosh y + \cosh x \sinh y \). This gives us \( \frac{e^{x+y} - e^{-(x+y)}}{2} = \frac{(e^x)e^y - (e^x)e^{-y}}{2} + \frac{(e^{-x})e^y + (e^{-x})e^{-y}}{2} \).

Step 3: Simplify The Expressions

Simplify the expressions by observing that \( e^x \cdot e^y = e^{x+y} \) and \( e^{-x} \cdot e^{-y} = e^{-(x+y)} \). Apply these to the equations and you get \( \frac{e^{x+y} - e^{-(x+y)}}{2} = \frac{e^{x+y} - e^{-(x+y)}}{2} \)

Key Concepts

Hyperbolic Identities

Hyperbolic functions are analogs of the well-known trigonometric functions, and they are used in various mathematical applications including hyperbolic geometry, calculus, and complex analysis.   The hyperbolic sine and cosine functions, denoted as \( \sinh \) and \( \cosh \) respectively, play a significant role in expanding and simplifying expressions, just like their trigonometric counterparts.
The main hyperbolic identities to remember are:

• \( \sinh^2 x + \cosh^2 x = \cosh 2x \)
• \( \sinh(x+y) = \sinh x \cosh y + \cosh x \sinh y \)
• \( \cosh(x+y) = \cosh x \cosh y + \sinh x \sinh y \)

These identities are useful when working with equations involving hyperbolic functions, especially when transforming between different expressions or verifying equivalencies. They have similar forms as trigonometric identities but differ in some sign conventions. This can sometimes trip up students new to these concepts, so careful attention is needed.

Trigonometric Identities

Trigonometric identities are equations that hold true for all values of the involved variables. They're invaluable tools in simplifying complex trigonometric equations and proving other mathematical expressions, much like hyperbolic identities do for their functions. Here are some key trigonometric identities:

• Pythagorean Identities: \( \sin^2 x + \cos^2 x = 1 \)
• Sum-to-Product Identities: \( \sin(x + y) = \sin x \cos y + \cos x \sin y \)
• Double Angle Identities: \( \cos(2x) = \cos^2 x - \sin^2 x \)

These are essential in calculus, physics, and engineering due to their ability to transform trigonometric expressions for easier manipulation or integration. Just like hyperbolic identities, these play a central role in deeper mathematical problem-solving and are worth mastering.

Exponential Functions

Exponential functions, defined as functions of the form \( f(x) = e^x \), are ubiquitous in mathematical modeling and analysis. These functions, due to their unique properties, are the building blocks of hyperbolic functions as well.

The exponential function \( e^x \) grows rapidly and is characterized by its constant proportion of growth rate to the value of the function itself. This property is crucial in understanding derivatives, integrals, and differential equations.

• The derivative of \( e^x \) is \( e^x \), which highlights its unique nature among functions.
• In solving hyperbolic functions, we often use exponential functions to rewrite the equations, such as \( \sinh x = \frac{e^x - e^{-x}}{2} \) and \( \cosh x = \frac{e^x + e^{-x}}{2} \).
• Exponential growth and decay model many natural phenomena, such as population growth and radioactive decay, making them highly relevant in real-world applications.

Understanding exponential functions is fundamental, not just in calculus, but in various scientific and engineering disciplines.