Chapter 13: Problem 2
(a) Determine \(\int\left(8 x^{3}+6 x^{2}-5 x+4\right) \mathrm{d} x\). (b) If \(I=\int\left(4 x^{3}-3 x^{2}+6 x-2\right) \mathrm{d} x\), determine the value of \(I\) when \(x=4\), given that when \(x=2, I=20\)
Short Answer
Expert verified
(a) \(\int = 2x^4 + 2x^3 - \frac{5}{2}x^2 + 4x + C\). (b) \(I = 236\) when \(x = 4\).
Step by step solution
01
Identify the Integrand for Part (a)
The integrand is the polynomial function \(8x^3 + 6x^2 - 5x + 4\). We will integrate this with respect to \(x\).
02
Integrate Term by Term
Integrate each term in the polynomial separately:- The integral of \(8x^3\) with respect to \(x\) is \(\frac{8}{4}x^4 = 2x^4\).- The integral of \(6x^2\) is \(\frac{6}{3}x^3 = 2x^3\).- The integral of \(-5x\) is \(-\frac{5}{2}x^2\).- The integral of \(4\) is \(4x\).
03
Combine Results with Constant of Integration
Combine the integrals of each term and add the constant of integration \(C\): \[ \int \left(8x^3 + 6x^2 - 5x + 4\right) \, dx = 2x^4 + 2x^3 - \frac{5}{2}x^2 + 4x + C \]
04
Identify Given Information for Part (b)
We have \(I = \int (4x^3 - 3x^2 + 6x - 2) \, dx\) and the initial condition \(I = 20\) when \(x = 2\), and need to find the value of \(I\) when \(x = 4\).
05
Integrate the Given Polynomial for Part (b)
Integrate each term for \(I\):- The integral of \(4x^3\) is \(x^4\).- The integral of \(-3x^2\) is \(-x^3\).- The integral of \(6x\) is \(3x^2\).- The integral of \(-2\) is \(-2x\).
06
Express the Integrated Function with Constant
Combine the results and include constant \(C\):\[ I = x^4 - x^3 + 3x^2 - 2x + C \]
07
Solve for Constant Using Initial Condition
Substitute \(x = 2\) into the integral:\[ I = 2^4 - 2^3 + 3(2)^2 - 2(2) + C = 20 \]Calculate: \[ 16 - 8 + 12 - 4 + C = 20 \]Simplify: \[ 16 + C = 20 \]\(C = 4\).
08
Calculate I for x = 4
Substitute \(x = 4\) into the function \(I = x^4 - x^3 + 3x^2 - 2x + 4\):\[ I = 4^4 - 4^3 + 3(4)^2 - 2(4) + 4 \]Calculate the value:\[ 256 - 64 + 48 - 8 + 4 = 236 \]
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Polynomial Integration
Polynomial integration is a fundamental process in calculus that involves finding the antiderivative of a polynomial function. The general rule is to integrate each term independently. - For a term with the form of a power, such as \( ax^n \), the integral is \( \frac{a}{n+1}x^{n+1} \). This means that we increase the exponent by one and divide by the new exponent.
- If you have a constant term, such as "4" in our example, its integral is simply that constant multiplied by \( x \), or "4x" in this scenario.
To integrate the polynomial in the exercise, we handled each term separately and applied these rules. For instance, \(8x^3\) became \(2x^4\) after integration, because \(\frac{8}{4}x^4 = 2x^4\). This step-by-step integration allows us to combine the results into a single antiderivative with ease.
- If you have a constant term, such as "4" in our example, its integral is simply that constant multiplied by \( x \), or "4x" in this scenario.
To integrate the polynomial in the exercise, we handled each term separately and applied these rules. For instance, \(8x^3\) became \(2x^4\) after integration, because \(\frac{8}{4}x^4 = 2x^4\). This step-by-step integration allows us to combine the results into a single antiderivative with ease.
Constant of Integration
Every time you integrate a function, you add a constant of integration, often denoted as \( C \), to account for the unknown initial starting point of the function's derivative. This constant is crucial, as it recognizes there are infinitely many antiderivatives that differ only by a constant.
- Consider integrating \(2x^4 + 2x^3 - \frac{5}{2}x^2 + 4x\) as detailed in the problem. The resulting expression becomes \(2x^4 + 2x^3 - \frac{5}{2}x^2 + 4x + C \).
- Without knowing initial conditions, \( C \) could be any real number, and thus the integral represents a family of curves.
By introducing the constant at the end of polynomial integration, we're preparing the function for possible interpretation under specific conditions. This will guide us in problem-solving, especially in practical situations where initial values are given.
- Consider integrating \(2x^4 + 2x^3 - \frac{5}{2}x^2 + 4x\) as detailed in the problem. The resulting expression becomes \(2x^4 + 2x^3 - \frac{5}{2}x^2 + 4x + C \).
- Without knowing initial conditions, \( C \) could be any real number, and thus the integral represents a family of curves.
By introducing the constant at the end of polynomial integration, we're preparing the function for possible interpretation under specific conditions. This will guide us in problem-solving, especially in practical situations where initial values are given.
Initial Condition
Initial conditions serve as vital additional information that allow us to solve for the constant of integration \( C \) in definite terms. When a specific value of the function is known at a particular point, we can find an exact solution to the integral expression.- For instance, we're given \( I = 20 \) when \( x = 2 \) in the example exercise. This tells us that when the variable is 2, the entire value of the integral results in 20.
- In mathematical terms, substituting \( x = 2 \) into the integrated expression enables us to solve for \( C \). Here,\[ 16 + C = 20 \] simplifies to \( C = 4 \).
Using the constant and initial condition lets us evaluate the integral accurately for other values, such as \( x = 4 \) in the problem. Consequently, this approach ensures our integrated function reflects the real-life scenarios or datasets accurately.
- In mathematical terms, substituting \( x = 2 \) into the integrated expression enables us to solve for \( C \). Here,\[ 16 + C = 20 \] simplifies to \( C = 4 \).
Using the constant and initial condition lets us evaluate the integral accurately for other values, such as \( x = 4 \) in the problem. Consequently, this approach ensures our integrated function reflects the real-life scenarios or datasets accurately.