Chapter 7: Problem 56
Find the area of the region between \(y=x \sin x\) and \(y=x\) for \(0 \leq x \leq \pi / 2\)
Short Answer
Expert verified
The area between the curves is \( \frac{\pi^2}{8} - 1 \).
Step by step solution
01
Identify the Intersection Points
To find the area between the curves, we first need to determine where they intersect. Set the equations equal: \[ x \sin x = x \]Divide by \(x\) (assuming \(x eq 0\)): \[ \sin x = 1 \]The solution in the given interval \( [0, \frac{\pi}{2}] \) is \( x = \frac{\pi}{2} \).Thus, the curves intersect at \( x = 0 \) and \( x = \frac{\pi}{2} \).
02
Set Up the Integral for the Area
The area between the curves from \(x = 0\) to \(x = \frac{\pi}{2}\) is found by integrating the difference between the functions:\[ A = \int_{0}^{\frac{\pi}{2}} \left( x - x \sin x \right) \, dx \]
03
Simplify the Integral
Simplify the integrand:\[ x - x \sin x = x(1 - \sin x) \]Thus, the integral simplifies to:\[ \int_{0}^{\frac{\pi}{2}} x(1 - \sin x) \, dx = \int_{0}^{\frac{\pi}{2}} x - x \sin x \, dx \]
04
Evaluate the Integral
Separate the integral into two parts:\[ \int_{0}^{\frac{\pi}{2}} x \, dx - \int_{0}^{\frac{\pi}{2}} x \sin x \, dx \]The first integral is straightforward:\[ \int_{0}^{\frac{\pi}{2}} x \, dx = \left[ \frac{x^2}{2} \right]_0^{\frac{\pi}{2}} = \frac{\pi^2}{8} \]The second integral requires integration by parts. Let:\( u = x \) and \( dv = \sin x \, dx \).Then, \( du = dx \) and \( v = -\cos x \). Applying integration by parts, \[ \int x \sin x \, dx = -x \cos x \Big|_{0}^{\frac{\pi}{2}} + \int \cos x \, dx \]\[ = -\left( \frac{\pi}{2} \cdot 0 - 0 \cdot (-1) \right) + \sin x \Big|_{0}^{\frac{\pi}{2}} \]\[ = -\left( \frac{\pi}{2} \cdot 0 \right) + \left( 1 - 0 \right) = 1 \]Hence,\[ \int_{0}^{\frac{\pi}{2}} x \sin x \, dx = 1 \]
05
Calculate the Total Area
The area is:\[ A = \frac{\pi^2}{8} - 1 \]
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Area between curves
Finding the area between curves is a core concept in Integral Calculus. The method involves plotting two functions on the same graph and identifying the region lying between them over a specific interval. This is often useful in physics and engineering to compute physical quantities such as distance or displacement. To find this area:
- Determine the points where the curves intersect. For the functions \( y = x \sin x \) and \( y = x \), you set them equal: \( x \sin x = x \) to find \( \sin x = 1 \). Thus, they intersect at points \( x = 0 \) and \( x = \frac{\pi}{2} \).
- The area between the curves from \( x = 0 \) to \( x = \frac{\pi}{2} \) is found by the integral of the difference between the curves: \( \int_{0}^{\frac{\pi}{2}} (x - x \sin x) \, dx \).
Integration by parts
Integration by parts is a technique applied to integrals where the integrand is a product of two functions. It is based on the product rule for differentiation and can be useful when other methods are impractical. The formula is:\[\int u \, dv = uv - \int v \, du\]To solve the integral \( \int x \sin x \, dx \):
- Choose \( u = x \), resulting in \( du = dx \).
- Choose \( dv = \sin x \, dx \), leading to \( v = -\cos x \).
Trigonometric integration
Trigonometric integration involves integrating expressions that include trigonometric functions. Such integrals are common and have many applications. In this context, we encounter it with the function \( x \sin x \). Often, these integrals require specific techniques like substitution or integration by parts.
When dealing with the trigonometric function \( \sin x \), remember:
When dealing with the trigonometric function \( \sin x \), remember:
- \( \int \sin x \, dx = -\cos x + C \)
- \( \int \cos x \, dx = \sin x + C \)