Chapter 54: Problem 991
Find the finite area above the \(\mathrm{x}\) -axis and under the curve \(y=x^{3}-9 x^{2}+23 x-15\)
Short Answer
Expert verified
The finite area above the x-axis and under the curve \(y=x^3 - 9x^2 + 23x - 15\) is 191 square units. To find this area, we first determined the x-intercepts (x = 1, x = 3, and x = 5) and then calculated the integral of the function in the intervals (x = 1 to x = 3 and x = 3 to x = 5).
Step by step solution
01
Determine the x-intercepts of the function
To find the x-intercepts of the function, set \(y = 0\) and solve for x. Therefore, we have:
\( 0 = x^3 - 9x^2 + 23x - 15 \)
We need to find the values of x where this equation holds.
02
Factor the polynomial
In order to find the x-intercepts, we can factor this polynomial. Factoring the polynomial helps us break it into simpler terms from which we can find the roots.
\( x^3 - 9x^2 + 23x - 15 = (x - 1)(x - 3)(x - 5) \)
Now, we can easily find the x-intercepts of the function:
\( x - 1 = 0 \Rightarrow x = 1 \)
\( x - 3 = 0 \Rightarrow x = 3 \)
\( x - 5 = 0 \Rightarrow x = 5 \)
The x-intercepts of the function are: x = 1, x = 3, and x = 5.
03
Determine the area under the curve
Since the function is over the \(\mathrm{x}\)-axis between x=1 and x=3, and between x=3 and x=5, we need to calculate the area under the curve in these intervals. We can achieve this by integrating the given function in each interval and finding the difference:
\( \int_{1}^{3} (x^3 - 9x^2 + 23x - 15)dx + \int_{3}^{5} (x^3 - 9x^2 + 23x - 15)dx\)
Now, we will integrate the function concerning x:
\( \int (x^3 - 9x^2 + 23x - 15)dx = \frac{1}{4}x^4 - 3x^3 + \frac{23}{2}x^2 - 15x + C \)
Now, calculate each integral:
\( \left[\frac{1}{4}(3)^4 - 3(3)^3 + \frac{23}{2}(3)^2 - 15(3)\right] - \left[\frac{1}{4}(1)^4 - 3(1)^3 + \frac{23}{2}(1)^2 - 15(1)\right] + \\
\left[\frac{1}{4}(5)^4 - 3(5)^3 + \frac{23}{2}(5)^2 - 15(5)\right] - \left[\frac{1}{4}(3)^4 - 3(3)^3 + \frac{23}{2}(3)^2 - 15(3)\right]\)
Plugging the given x values into the integrated function and evaluating the expression gives:
\[ \left[-72\right] - \left[-34\right] + \left[-225\right] - \left[-72\right] = 38 + 153 = 191 \]
04
Interpret the results
Now that we have calculated the value of the integral, we can interpret its meaning. The total area under the curve and above the \(\mathrm{x}\)-axis for the intervals we have considered (x = 1 to x = 3 and x = 3 to x = 5) is 191 square units.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Polynomial Functions
Polynomial functions are mathematical expressions involving a sum of powers of one or more variables multiplied by coefficients. Consider the polynomial function provided in the exercise:
- The polynomial: \( y = x^3 - 9x^2 + 23x - 15 \)
- The degree of this polynomial, which is the highest power of the variable \( x \), is 3, classifying it as a cubic polynomial.
- Each term in this polynomial consists of a coefficient (such as 1, -9, 23, and -15) and a variable \( x \) raised to an exponent.
Area Under a Curve
Finding the area under a curve is a fundamental problem in calculus, frequently solved using definite integrals. This exercise involves determining such an area:
- The specific area to find here is between the curve of the polynomial function and the x-axis, limited to an interval on the x-axis.
- The intervals examined in this exercise are \([1, 3]\) and \([3, 5]\), which correspond to the x-intercepts or roots of the polynomial function.
- In practice, calculating the area under a curve involves setting up and solving definite integrals.
Integration Techniques
Integration is a powerful technique used to calculate areas, among other things, by reversing the process of differentiation. In the solution provided, indefinite and definite integration strategies are used:
- The process starts with integrating the polynomial function \(x^3 - 9x^2 + 23x - 15\).
- By finding the antiderivative, the integral expression becomes \( \frac{1}{4}x^4 - 3x^3 + \frac{23}{2}x^2 - 15x \), representing the indefinite integral.
- The definite integral evaluates this antiderivative at specific boundaries (limits of integration), particularly for each part of the interval \([1, 3]\) and \([3, 5]\).
Roots of Polynomials
The roots of a polynomial are the x-values where the polynomial equals zero (\(f(x) = 0\)). Finding these roots:
- Involves setting the polynomial equation \( x^3 - 9x^2 + 23x - 15 = 0 \) and solving for \( x \).
- In this exercise, the polynomial is factorable into \((x - 1)(x - 3)(x - 5)\), which helps identify the x-intercepts.
- The roots are \( x = 1, 3, \) and \( 5 \), which are key to marking boundaries for the integral calculation.