Problem 1 - Thomas Calculus in SI Units

Question

Gives a value of \(\sinh x\) or \(\cosh x .\) Use the definitions and the identity \(\cosh ^{2} x-\sinh ^{2} x=1\) to find the values of the remaining five hyperbolic functions. $$\sinh x=-\frac{3}{4}$$

Step-by-Step Solution

Answer

The remaining hyperbolic functions are: \( \cosh x = \frac{5}{4} \), \( \tanh x = -\frac{3}{5} \), \( \text{csch} x = -\frac{4}{3} \), \( \text{sech} x = \frac{4}{5} \), \( \text{coth} x = -\frac{5}{3} \).

Step 1: Use the hyperbolic identity

We know the hyperbolic identity is \( \cosh^2 x - \sinh^2 x = 1 \). Substitute \( \sinh x = -\frac{3}{4} \) into the identity to find \( \cosh x \).

Step 2: Calculate \( \cosh^2 x \)

Substitute \( \sinh x = -\frac{3}{4} \) into the hyperbolic identity: \[ \cosh^2 x - \left(-\frac{3}{4}\right)^2 = 1 \]. This simplifies to \( \cosh^2 x - \frac{9}{16} = 1 \).

Step 3: Solve for \( \cosh^2 x \)

Rearrange the equation: \( \cosh^2 x = 1 + \frac{9}{16} \). Convert \(1\) to a fraction: \( \cosh^2 x = \frac{16}{16} + \frac{9}{16} = \frac{25}{16} \).

Step 4: Determine \( \cosh x \)

Solve for \( \cosh x \), taking the square root of both sides: \( \cosh x = \pm \frac{5}{4} \). By convention, \( \cosh x \) is non-negative, so \( \cosh x = \frac{5}{4} \).

Step 5: Calculate the remaining hyperbolic functions

Using \( \sinh x = -\frac{3}{4} \) and \( \cosh x = \frac{5}{4} \), calculate the remaining functions:- \( \tanh x = \frac{\sinh x}{\cosh x} = \frac{-\frac{3}{4}}{\frac{5}{4}} = -\frac{3}{5} \)- \( \text{csch} x = \frac{1}{\sinh x} = -\frac{4}{3} \)- \( \text{sech} x = \frac{1}{\cosh x} = \frac{4}{5} \)- \( \text{coth} x = \frac{\cosh x}{\sinh x} = -\frac{5}{3} \)

Key Concepts

sinh function

The hyperbolic sine, often written as \( \sinh x \), is an important hyperbolic function used in various mathematical applications. Like trigonometric functions, hyperbolic functions define relations using a hyperbola, and the \( \sinh \) function is akin to the sine function for circular motion. It is defined by the equation:\[ \sinh x = \frac{e^x - e^{-x}}{2} \]where \( e \) represents Euler's number, the base of the natural logarithm.
In the given problem, \( \sinh x = -\frac{3}{4} \), it implies that the output of the \( \sinh \) function at a particular \( x \) value is \(-\frac{3}{4}\). Similarly to understanding a sine wave, you can visualize \( \sinh \) as a wave that passes through the origin, extending indefinitely upwards and downwards. The negative value \(-\frac{3}{4}\) merely tells us that we are looking at a point below the x-axis in this wave.

• The hyperbolic sine is unbounded, meaning it can take on any real value, from negative infinity to positive infinity.
• It's an odd function, meaning \( \sinh(-x) = -\sinh(x) \).
• In practical terms, hyperbolic functions are well-suited for modeling realistic structures like catenary (shapes of hanging cables).

cosh function

The hyperbolic cosine, denoted as \( \cosh x \), plays a crucial role in hyperbolic mathematics, similar to the cosine function in trigonometry. It is defined by the formula:\[ \cosh x = \frac{e^x + e^{-x}}{2} \]where \( e \) is Euler's number.
\( \cosh x \) is always positive or zero, unlike \( \sinh x \), and reaches its minimum value of 1 when \( x = 0 \). It never falls below 1, making it a crucial function for symmetric structures in hyperbolic geometry.

• Ordinarily drawn, \( \cosh x \) resembles the right half of a catenary curve, never dipping below the x-axis.
• It’s an even function, with the property \( \cosh(-x) = \cosh(x) \).
• Common applications include calculating distances in hyperbolic planes and solving physics problems involving hyperbolic motion.

Given the problem's scenario with \( \sinh x = -\frac{3}{4} \), we use the hyperbolic identity to solve for \( \cosh x \). After substituting values and performing arithmetic, we find that \( \cosh x = \frac{5}{4} \).
This means the curve at this specific \( x \) value is \( \frac{5}{4} \) units away from its minimum point, above the x-axis.

hyperbolic identity

The hyperbolic identity \( \cosh^2 x - \sinh^2 x = 1 \) is a fundamental relationship that ties together the hyperbolic sine and cosine functions, analogous to the well-known Pythagorean identity in trigonometry: \( \cos^2 x + \sin^2 x = 1 \). This identity is key in solving problems involving hyperbolic functions, as seen in this exercise.
By substituting the given \( \sinh x = -\frac{3}{4} \) into the identity, we were able to solve for \( \cosh x \). The transformation into a solvable equation allows for finding one function if the other is known, assuming the correct values are selected according to the problem's constraints.

• This hyperbolic identity is critical for simplifying and solving equations involving hyperbolic functions, making it indispensable in both theoretical and applied mathematics.
• Unlike the Pythagorean identity, which involves addition, the hyperbolic identity uses subtraction, highlighting the unique behavior of hyperbolic functions.

The use of this identity ensures that any pair of \( \sinh x \) and \( \cosh x \) values satisfy this relationship, providing consistency in calculations involving derivatives and integrals of hyperbolic functions.