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Find \(d y / d x\) if \(y=\cos (6 x+2)\) by writing \(y\) as a composite with \(\begin{array}{ll}{\text { (a) } y=\cos u} & {\text { and } u=6 x+2} \\\ {\text { (b) } y=\cos 2 u} & {\text { and } \quad u=3 x+1}\end{array}\)

Short Answer

Expert verified
The derivative of the function \(y = \cos(6x + 2)\) is \(-6\sin(6x + 2)\) whether written as composite function (a) or (b)

Step by step solution

01

Find the derivative using first composite function

We are given that \(y = \cos u\) and \(u = 6x + 2\). The chain rule states that \(dy/dx = dy/du * du/dx\). So, differentiating \(y\) with respect to \(u\) gives \(-\sin u\), and differentiating \(u\) with respect to \(x\) gives 6. Therefore, \(dy/dx = -\sin u * 6 = -6\sin u\).
02

Substitute \(u\) back into the function

Replace \(u\) with \(6x + 2\) to write the derivative in terms of \(x\). So, \(dy/dx = -6\sin(6x + 2)\). This is the derivative of the given function when \(y\) is written with composite function (a).
03

Find the derivative using second composite function

Now, we are given \(y = \cos 2u\) and \(u= 3x + 1\). Applying the chain rule again, we first differentiate \(y\) with respect to \(u\) to get \(-2\sin 2u\) and differentiate \(u\) with respect to \(x\) to get 3. So, \(dy/dx = -2\sin 2u * 3 = -6\sin 2u\).
04

Substitute \(u\) back into the function

Again, replace \(u\) with \(3x + 1\) to write the derivative in terms of \(x\). So, \(dy/dx = -6\sin(2(3x + 1)) = -6\sin(6x + 2)\). This is the derivative of the given function when \(y\) is written with composite function (b).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Composite Functions
Composite functions are intriguing constructs in mathematics where one function is applied to the result of another function. In our exercise, \(y = \cos(6x + 2)\), this can be seen by expressing it as \(y = \cos(u)\) and \(u = 6x + 2\). Here, \(y\) is a composite function because the output of \(u\) (which is in terms of \(x\)) is fed into the \(\cos\) function.

To simplify such functions, breaking them down helps you see the inner workings of the chain rule. The inner function \(u\) is calculated first, and its output is used in the outer function \(\cos\).
  • The idea is to handle complex expressions more manageably.
  • It allows for calculating derivatives or other operations piece by piece, focusing first on the inner function results, followed by the outer function application.
By viewing it as a composite, it becomes easy to see the function in stages and apply rules like the chain rule effectively.
Derivatives
Derivatives form the backbone of calculus, signifying the rate at which a function changes. In the context of this exercise, the chain rule is applied to find the derivative of a composite function—a task involving deriving each layer of the composition separately.

The chain rule in calculus provides a method for differentiating composite functions. It states: \(\frac{dy}{dx} = \frac{dy}{du} \cdot \frac{du}{dx}\). This implies:
  • Calculate the derivative of the outer function \(y\) in terms of the inner variable \(u\).
  • Find the derivative of the inner function \(u\) with respect to \(x\).
  • Combine these derivatives through multiplication.
In our specific exercise, the derivative of \(y = \cos(u)\) concerning \(u\) is \(-\sin(u)\), and the derivative of \(u = 6x + 2\) with respect to \(x\) is \(6\). Multiplying these gives the derivative of the whole composite function. Mastering derivatives, especially through techniques like the chain rule, equips us to handle complex mathematical situations.
Trigonometric Functions
Trigonometric functions like sine, cosine, and tangent are pivotal in calculus, representing periodic phenomena and circle-related functions. When dealing with these functions in calculus, their derivatives become significant, allowing us to analyze rates of change and slopes of curves.

In our exercise, the function involves the cosine trigonometric function where \(y = \cos(6x + 2)\). The process of finding its derivative inherently ties back to understanding the behavior of these trigonometric functions:
  • The derivative of \(\cos(u)\) is \(-\sin(u)\), reflecting the cosine's periodic properties.
  • These derivatives reveal oscillations and patterns in trigonometric graphs.
Understanding the derivatives of trigonometric functions is crucial, especially in physics and engineering applications, where wave behavior and circular motion are analyzed. In essence, these functions and their derivatives form an integral part of both mathematical theory and its practical applications.

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Most popular questions from this chapter

Generating the Birthday Probabilities Example 5 of this section concerns the probability that, in a group of \(n\) people, at least two people will share a common birthday. You can generate these probabilities on your calculator for values of \(n\) from 1 to \(365 .\) Step 1: Set the values of \(N\) and \(P\) to zero: Step \(2 :\) Type in this single, multi-step command: Now each time you press the ENTER key, the command will print a new value of \(N(\) the number of people in the room) alongside \(P\) (the probability that at least two of them share a common birthday): If you have some experience with probability, try to answer the following questions without looking at the table: (a) If there are three people in the room, what is the probability that they all have different birthdays? (Assume that there are 365 possible birthdays, all of them equally likely.) (b) If there are three people in the room, what is the probability that at least two of them share a common birthday? (c) Explain how you can use the answer in part (b) to find the probability of a shared birthday when there are four people in the room. (This is how the calculator statement in Step 2 generates the probabilities.) (d) Is it reasonable to assume that all calendar dates are equally likely birthdays? Explain your answer.

The Derivative of sin 2\(x\) Graph the function \(y=2 \cos 2 x\) for \(-2 \leq x \leq 3.5 .\) Then, on the same screen, graph $$\quad y=\frac{\sin 2(x+h)-\sin 2 x}{h}$$ for \(h=1.0,0.5,\) and \(0.2 .\) Experiment with other values of \(h,\) including negative values. What do you see happening as \(h \rightarrow 0 ?\) Explain this behavior.

Multiple Choice Which of the following is the domain of \(f^{\prime}(x)\) if \(f(x)=\log _{2}(x+3) ? \quad\) (A) \(x<-3 \quad\) (B) \(x \leq 3 \quad\) (C) \(x \neq-3 \quad\) (D) \(x>-3\) (E) \(x \geq-3\)

In Exercises \(41-46,\) find (a) the right end behavior model, (b) the left end behavior model, and (c) any horizontal tangents for the function if they exist. $$y=\cos ^{-1} x$$

At what point on the graph of \(y=3^{x}+1\) is the tangent line parallel to the line \(y=5 x-1 ?\)

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