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: $$\cosh x+\sinh x=e^{x}$$

Step 1: Writing down the definitions of the hyperbolic functions

First, let us recall the definitions of hyperbolic cosine and sine functions: $$\cosh x=\frac{e^{x}+e^{-x}}{2}$$ $$\sinh x=\frac{e^{x}-e^{-x}}{2}$$

Step 2: Summing the hyperbolic functions

Now, we need to find the sum of \(\cosh x\) and \(\sinh x\): $$\cosh x+\sinh x = \frac{e^{x}+e^{-x}}{2} + \frac{e^{x}-e^{-x}}{2}$$

Step 3: Simplifying the expression

Given that we have a common denominator of 2, let's combine the numerators: $$\cosh x+\sinh x =\frac{(e^{x}+e^{-x}) + (e^{x}-e^{-x})}{2}$$ $$\cosh x+\sinh x =\frac{(e^{x}+e^{x}) + (e^{-x}-e^{-x})}{2}$$ Notice that \(e^{-x}\) and \(-e^{-x}\) cancel out, and we are left with: $$\cosh x+\sinh x =\frac{2e^{x}}{2}$$

Step 4: Final simplification

Finally, we simplify the expression by canceling out the common factor of 2: $$\cosh x+\sinh x =e^{x}$$ We have successfully verified the identity by showing that the left-hand side, \(\cosh x+\sinh x\), is indeed equal to the right-hand side, \(e^{x}\).

Key Concepts

Hyperbolic Cosine

The hyperbolic cosine, denoted as \( \cosh x \), is one of the primary hyperbolic functions used in mathematics, and it has similarities to the familiar trigonometric cosine function. However, instead of relating to circles, hyperbolic functions are connected to hyperbolas.

To define \( \cosh x \), we use exponential functions: \[ \cosh x = \frac{e^{x} + e^{-x}}{2} \] This is the average of the exponential function \( e^{x} \) and its reciprocal. Hyperbolic cosine is an even function, meaning that \( \cosh(-x) = \cosh(x) \), which shows that its graph is symmetric with respect to the y-axis.

Hyperbolic Sine

In parallel to hyperbolic cosine, the hyperbolic sine, \( \sinh x \), is part of the hyperbolic function family that mirrors aspects of the classic trigonometric sine.

The function is defined by the following relation to exponential functions: \[ \sinh x = \frac{e^{x} - e^{-x}}{2} \] Unlike \( \cosh x \), the function \( \sinh x \) is odd, which implies \( \sinh(-x) = -\sinh(x) \) and the function graph is symmetric with respect to the origin. Hyperbolic sine is widely used in various areas, such as the description of catenary curves (the shape of a hanging chain) or in the theory of special relativity.

Exponential Functions

Exponential functions are the backbone of many mathematical concepts, including hyperbolic functions. An exponential function is of the form \( e^{x} \), where \( e \) is the base, an irrational constant approximately equal to 2.71828, and \( x \) is the exponent.

These functions are characterized by their property \( e^{x+y} = e^{x}e^{y} \) and depict various processes such as growth and decay in biology, population dynamics, and compound interest in finance. The inverse of the exponential function is the natural logarithm, defined as \( \ln(x) \). Exponential functions are continuous and infinitely differentiable, making them a crucial element in calculus.

Mathematical Proofs

Mathematical proofs are logical arguments that establish the truth of mathematical statements. They involve a sequence of deductive reasoning steps from known facts and previously established results to reach a conclusion.

Proofs are essential in mathematics because they provide a means of ensuring that theories and propositions are sound and reliable. Different types of proofs include direct proofs, indirect proofs (proof by contradiction), and proof by induction.

In our exercise, we used direct proof to establish the identity \( \cosh x + \sinh x = e^{x} \). By carefully applying the definitions and properties of hyperbolic functions and simplifying, we arrived at the exponential function that confirmed our identity as true. This approach not only validates the statement but also reinforces understanding of the interconnectedness of mathematical concepts.