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Approximating Area with the Midpoint Rule In Exercises \(61-64,\) use the Midpoint Rule with \(n=4\) to approximate the area of the region bounded by the graph of the function and the \(x\) -axis over the given interval. $$ f(x)=x^{2}+4 x, \quad[0,4] $$

Short Answer

Expert verified
The approximation of the area under the curve \(f(x)=x^{2}+4x\) on the interval [0,4] using the Midpoint Rule with 4 subintervals is 51.

Step by step solution

01

Identify the interval and number of subintervals

We are given the interval as \([0,4]\) and number of subintervals \(n=4\). Each subinterval thus has a width of \(\Delta x = \frac{(b - a)}{n} = 1\). In this case, \(a=0\) and \(b=4\).
02

Find the midpoints

To find the midpoints, we add half of the interval width, \(\Delta x=1\), to the start of each subinterval. Therefore, the midpoints are \(x_{1}=0.5\), \(x_{2}=1.5\), \(x_{3}=2.5\), and \(x_{4}=3.5\).
03

Evaluate the function at each midpoint

Now that we have our midpoints, we can evaluate \(f(x)\) at each midpoint. We get \(f(x_{1})=2.25\), \(f(x_{2})=7.25\), \(f(x_{3})=15.25\), and \(f(x_{4})=26.25\). This is done by substituting \(x_{i}\) into \(f(x)=x^{2}+4x\).
04

Compute the approximation

We now apply the Midpoint rule approximation formula which is \((b - a) \times \frac{sum \, of \, midpoints}{n}\). We already know from Step 1 that \(b-a=4\). The 'sum of midpoints' refers to the sum of function values at the midpoints which we find in Step 3. The sum is \(2.25 + 7.25 + 15.25 + 26.25 = 51\). Therefore, the approximation of the area is \((4) \times \frac{51}{4} = 51\).

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Key Concepts

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

Approximating Area
Approximating the area under a curve can be a complex task, especially when dealing with functions that are not easily integrable analytically. The Midpoint Rule provides a simple yet effective way to find an approximate area. The idea revolves around breaking down the entire interval into smaller sections known as subintervals. Instead of focusing on the entire curve, we approximate using the middle of these subintervals.

By evaluating the function at the midpoint of each subinterval, we can estimate the area under the curve by summing the areas of rectangles formed. This approach simplifies the problem considerably, turning a complex curve into manageable segments.
  • The formula for a midpoint approximation is: \[ \text{Approximate Area} = \sum f(m_i) \cdot \Delta x \]
  • Here, \(m_i\) represents the midpoints, and \(\Delta x\) is the width of each subinterval.
This method is especially useful for quick estimates where precision takes a back seat, but efficiency and simplicity are prioritized.
Definite Integrals
A definite integral represents the exact area under a curve over a given interval. In calculus, the definite integral of a function \(f(x)\) from \(a\) to \(b\) gives a precise measure of the accumulated quantity or area.

The expression is typically written as: \[ \int_{a}^{b} f(x) \, dx \] This notation indicates the summation of infinitesimal elements of area over the interval. Calculating definite integrals analytically involves finding antiderivatives, which can sometimes be challenging or impossible.
  • This is where numerical integration methods, like the Midpoint Rule, come into play, providing a practical alternative to arrive at an approximate solution.
Even though the result is an estimate, when carefully executed, it can closely mimic the exact area given by the definite integral.
Numerical Integration
Numerical integration is the field of mathematics where we seek approximate solutions to integrals that are difficult or impossible to solve analytically. Often, we face functions that have no elementary antiderivatives. In such cases, numerical methods save the day.

These techniques are based on dividing a given interval into smaller segments and using simple calculations to estimate the integral. The Midpoint Rule is one such method, along with others like the Trapezoidal Rule and Simpson's Rule. Each has its own strengths and levels of precision.
  • These methods are crucial when working with real-world data, especially in engineering and physics, where exact solutions are not always practical.
  • By using numerical integration, we can handle complex functions that model real phenomena efficiently and accurately.
Though approximate, these calculations offer valuable insights and solutions extending beyond theoretical mathematics.
Subintervals
Subintervals are smaller pieces that make up the entire interval of interest when applying numerical integration methods. Let's say we wish to approximate the area under a curve between points \(a\) and \(b\). We then divide this range into several subintervals of equal width.

Each subinterval has a width denoted as \(\Delta x\), determined by the formula: \(\Delta x = \frac{b-a}{n}\), where \(n\) is the number of subintervals. By choosing more subintervals (higher \(n\)), we increase the approximation accuracy.
  • The midpoint of each subinterval is used in the Midpoint Rule, where it serves as the representative point for calculations.
  • Subintervals simplify the problem by allowing us to focus on small segments rather than the entire curve at once.
This step-wise approach makes the method efficient and more manageable, particularly for complex functions.

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Most popular questions from this chapter

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Approximating Area with the Midpoint Rule In Exercises \(61-64,\) use the Midpoint Rule with \(n=4\) to approximate the area of the region bounded by the graph of the function and the \(x\) -axis over the given interval. $$ f(x)=\cos x, \quad\left[0, \frac{\pi}{2}\right] $$

The function \(f(x)\) is concave upward on the interval \([0,2]\) and the function \(g(x)\) is concave downward on the interval \([0,2]\) . (a) Using the Trapezoidal Rule with \(n=4\) , which integral would be overestimated? Which integral would be underestimated? Explain your reasoning. (b) Which rule would you use for more accurate approximations of \(\int_{0}^{2} f(x) d x\) and \(\int_{0}^{2} g(x) d x,\) the Trapezoidal Rule or Simpson's Rule? Explain your reasoning.

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