Chapter 6: Problem 37
Let \(g(x)=x^{3}-4 x+6 .\) Find \(f(x)\) so that \(f^{\prime}(x)=g^{\prime}(x)\) and \(f(1)=2\).
Short Answer
Expert verified
\( f(x) = x^3 - 4x + 5 \)
Step by step solution
01
Find the derivative of g(x)
First, we need to find the derivative of the function \( g(x) = x^3 - 4x + 6 \). The derivative \( g'(x) \) is obtained by applying the power rule to each term in the polynomial:- The derivative of \( x^3 \) is \( 3x^2 \).- The derivative of \( -4x \) is \( -4 \).- The derivative of the constant term \( 6 \) is \( 0 \).Thus, \( g'(x) = 3x^2 - 4 \).
02
Assign f'(x) equal to g'(x)
Given that \( f'(x) = g'(x) \), we have from Step 1:\[ f'(x) = 3x^2 - 4 \]
03
Integrate f'(x) to find f(x)
To find \( f(x) \), we integrate \( f'(x) \):\[ \int (3x^2 - 4) \, dx \]- The integral of \( 3x^2 \) is \( x^3 \).- The integral of \( -4 \) is \( -4x \).Thus, \( f(x) = x^3 - 4x + C \), where \( C \) is the integration constant.
04
Solve for the constant C using f(1)=2
We use the given condition \( f(1) = 2 \) to determine the constant \( C \).Substitute \( x = 1 \) into the equation \( f(x) = x^3 - 4x + C \):\[ f(1) = 1^3 - 4(1) + C = 2 \]Simplifying gives:\[ 1 - 4 + C = 2 \]\[ -3 + C = 2 \]Solving for \( C \):\[ C = 5 \]
05
Conclude f(x)
Now that we have the constant \( C \), we can write the function:\( f(x) = x^3 - 4x + 5 \).
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
The Derivative
In calculus, the derivative represents how a function changes as its input changes. For a given function, its derivative provides the rate of change or slope of the function at any point. In this case, we focus on the function \( g(x) = x^3 - 4x + 6 \). To find the derivative, \( g'(x) \), we apply the power rule to each term.
- The derivative of \( x^3 \) is determined by multiplying 3 (the current power) by the coefficient (which is 1) and reducing the power by one, giving us \( 3x^2 \).
- For the term \( -4x \), the derivative is simply the constant factor \(-4\) (since the power of \( x \) is 1).
- The derivative of a constant, such as 6, is zero since a constant term does not change.
Integration
Integration is the inverse process of differentiation. When we integrate, we're looking for a function that, when differentiated, gives us the original function's rate of change. In this exercise, we are given \( f'(x) = 3x^2 - 4 \) and need to determine \( f(x) \) by integrating.
- The integral of \( 3x^2 \) can be found by increasing the power of \( x \) by one to get \( x^3 \), and dividing by the new power, resulting in \( x^3 \).
- The integral of a constant, such as \( -4 \), is \( -4x \) because we essentially multiply by \( x^1/1 \).
Power Rule
The power rule is a cornerstone of calculus, making the process of finding derivatives straightforward by a specific formula. It states that the derivative of \( x^n \) is \( nx^{n-1} \). This rule is not only useful but essential when dealing with polynomials, as it can be applied to each term independently.
- In our original function \( g(x) = x^3 - 4x + 6 \), the power rule allows us to say the derivative of \( x^3 \) is \( 3x^2 \), simplifying what would otherwise be a complex process.
- Furthermore, understanding the power rule facilitates ease in re-integrating polynomials to find functions from their derivatives, as seen when determining \( f(x) \).
Constant of Integration
The constant of integration, \( C \), is an integral part of indefinite integration. Because derivatives of constant terms are zero, when we work backwards to find a function from its rate of change (its derivative), we must include an arbitrary constant. This "+C" accounts for any constant shift that occurred when differentiating.
- In our case, once we found \( f(x) = x^3 - 4x + C \), it was crucial to identify \( C \) by using the condition \( f(1) = 2 \).
- By substituting \( x = 1 \) and \( f(x) = 2 \) into this form, we found \( 1^3 - 4 \times 1 + C = 2 \), and hence solved for \( C = 5 \).