Problem 42 - Calculus Early Transcendentals

Question

Let \(f(x)=\sin x .\) Find all positive integers \(n\) for which \(f^{(n)}(x)=\sin x\)

Step-by-Step Solution

Answer

All positive integers \(n\) that are multiples of 4 (e.g., 4, 8, 12, ...) make \(f^{(n)}(x) = \sin x\).

Step 1: Understand the Problem

We are given the function \(f(x) = \sin x\) and required to find positive integers \(n\) for which the \(n\)-th derivative \(f^{(n)}(x)\) is equal to the original function \(\sin x\).

Step 2: Identify the Pattern of Derivatives

Compute the first few derivatives of \(f(x) = \sin x\) to recognize any patterns: - \(f'(x) = \cos x\)- \(f''(x) = -\sin x\)- \(f'''(x) = -\cos x\)- \(f^{(4)}(x) = \sin x\)Notice the derivatives cycle every four steps: \(\sin x, \cos x, -\sin x, -\cos x\).

Step 3: Find the Cycle Length

The cycle of derivatives repeats every 4 derivatives: \(\sin x, \cos x, -\sin x, -\cos x, \sin x, \ldots\). This suggests that for every 4th derivative, we return to the same function.

Step 4: Determine All Suitable Values of n

Since the 4th derivative returns to \(\sin x\), we find that all positive integers \(n = 4k\), where \(k\) is a positive integer, will satisfy \(f^{(n)}(x) = \sin x\). This means \(n\) can be 4, 8, 12, etc.

Key Concepts

Trigonometric functions

Trigonometric functions, like sine (\(\sin x\)), are fundamental in calculus. These functions relate the angles and ratios of triangles, and are pivotal in oscillation and wave studies. - Sine, cosine (\(\cos x\)), and tangent are the primary trigonometric functions.- They display periodic behavior, repeating their values in regular intervals.Sine, for instance, has a period of \(2\pi\), demonstrating its values repeat every \(2\pi\) units. Trigonometric functions offer smooth and continuous curves, making them integral in modeling natural phenomena like sound and light waves. Understanding these functions is crucial for solving advanced calculus problems, particularly when derivatives are involved.

Derivative cycles

In calculus, derivatives indicate how a function changes. When calculating derivatives of trigonometric functions, we observe derivative cycles. These cycles reveal straightforward patterns.For the function \(f(x) = \sin x\):- The first derivative is \(f'(x) = \cos x\).- The second derivative is \(f''(x) = -\sin x\).- Continuing further, \(f'''(x) = -\cos x\).- Finally, the fourth derivative returns us to the original function, \(f^{(4)}(x) = \sin x\).This cycle of derivatives thus repeats every four functions. Recognizing this cyclic behavior simplifies problem-solving because it helps predict the nature of higher-order derivatives. Each function has a distinct cycle length in derivatives, an essential insight for tackling calculus questions efficiently.

Calculus problem solving

Probleming solving in calculus involves understanding the patterns and relationships within functions and their derivatives. By identifying cycles and patterns, we solve problems with greater ease and accuracy.For the given function \(f(x) = \sin x\), finding all positive integers \(n\) for which \(f^{(n)}(x) = \sin x\) requires recognizing the periodic derivative cycle. - Notice that every fourth derivative returns to the initial form, meaning for every cycle (\(n = 4k\), where \(k\) is a positive integer), we get back \(\sin x\).- Thus, potential values of \(n\) are 4, 8, 12, etc.When approaching calculus problems, identifying such patterns and cycles can lead to quicker and more accurate solutions, minimizing computational workload. Mastering these techniques is integral for students navigating higher-order derivatives and complex calculus scenarios.