Bisection Method: Definition, Steps & Solved Examples

The bisection method is a simple way to find solutions to equations with only one unknown. It's a popular technique in mathematics for solving transcendental equations. 

In this mathematics article, we will delve into the bisection method and provide detailed explanations and examples to help you understand and apply it effectively.

What is the Bisection Method?

The Bisection Method is a simple way to find the root (or solution) of an equation when it is hard to solve it directly. It is used to find where a function becomes zero — in other words, where it crosses the x-axis. To use this method, we start with two numbers, a and b, such that the function values at these points have opposite signs (one positive and one negative). This tells us there is a root between them.

We then find the middle point of the interval and check the value of the function there. Based on the sign at the midpoint, we decide whether to look in the left or right half of the interval next. This process is repeated again and again until we get close enough to the actual root.

The Bisection Method is slow but very reliable and always works when the function is continuous and the initial values are chosen correctly.

The Bisection Method is a simple way to find the solution (root) of a polynomial equation. It is used when we know the solution lies between two numbers, say a and b. This method works only if the function is continuous and the values at a and b have opposite signs — one positive and one negative. This means the function crosses the x-axis somewhere between a and b.

The method keeps dividing the interval into two equal parts. In each step, it checks the midpoint of the interval and sees if the function becomes zero there. If not, it looks at the sign of the function at the midpoint and decides whether to move to the left half or the right half. This process is repeated until we get very close to the actual root.

The Bisection Method is also called the interval halving method, binary search method, or dichotomy method. It is slow but very reliable and always works if the function meets the required conditions. It’s a great method when you want a safe and steady way to find where a function equals zero.

Bisection Method Explanation

The bisection method can be used to solve the equation for a real variable , given that is a continuous function defined on the interval and and have opposite signs.

The process involves dividing the interval into two equal parts by calculating the midpoint, denoted as , and evaluating the function at that position. If is not the root, there exist two potential outcomes.

• and exhibit different signs and enclose a root within their range.
• and display different signs and enclose a root within their range.

A sub-interval is selected as the new interval for the subsequent step, and this repetition continues until the interval reaches a suitable level of smallness.

If the signs of and differ, then replaces . When the signs of and differ, replaces . If equals zero, is considered the solution, and the process concludes.

How it works:

You start with two points, say a and b, where f(a) and f(b) have opposite signs. You then find the midpoint m = (a + b)/2 and check the value of f(m).

If f(m) is zero, you’ve found the root.

If not, check whether the sign of f(m) is the same as f(a) or f(b).

Replace the endpoint that has the same sign as f(m) with m and repeat the process.

This method keeps narrowing down the interval until the value is very close to the actual root. It’s slow but always gives the correct result if the function is continuous and changes sign in the interval.

The Bisection Method is widely used in mathematics and engineering for solving equations because it is easy to understand and apply.

Bisection Method Algorithm

To find the solution for a continuous function, follow these steps:

Consider a continuous function ,

Step 1: Find two points, and , where is smaller than , and the product of and is negative.

Step 2: Calculate the midpoint, , between and .

Step 3: If equals , then is the root of the function. If not, proceed to the next step.

Step 4: Divide the interval :

• If the product of and is negative, there exists a root between and .
• If the product of and is negative, there exists a root between and .

Step 5: Repeat the above three steps until equals .

In the above figure, the initial four stages of the bisection method involve taking certain steps. The presence of a significant blue dot denotes the root that has been discovered through four iterations. To achieve a higher level of accuracy, one can continue to repeat the aforementioned steps, adjusting the level of precision based on specific needs.

The bisection method is an approximation technique used to find the roots of an equation by repeatedly splitting the interval. This method keeps dividing the interval until a very narrow interval is obtained.

Advantages and Limitations of the Bisection Method

The Bisection Method offers several advantages:

• Guaranteed convergence: Unlike some other methods, the Bisection Method guarantees convergence to a root within the initial interval.
• Simplicity and ease of implementation: The method's straightforward nature makes it accessible to both beginners and experienced practitioners.
• Robustness: The Bisection Method can handle various types of equations, including nonlinear and transcendental functions.

However, the method also has limitations:

• Dependence on an initial interval: Selecting a suitable interval containing a root is crucial for successful implementation.
• Slow convergence: The Bisection Method converges at a linear rate, which can be slower compared to methods like Newton-Raphson.
• Inability to handle multiple roots: If an equation has multiple roots within the same interval, the Bisection Method may only find one of them.

Application of Bisection Method

The Bisection Method finds applications in diverse fields, including:

• Engineering and physics: Solving equations related to mechanics, fluid dynamics, and electrical circuits.
• Financial and economic modeling: Computing interest rates, asset pricing, and optimization problems.
• Biological and ecological systems: Analyzing population dynamics, ecological modeling, and biochemical reactions.

Tips and Tricks on Bisection Method

Here are some tips and tricks for efficient implementation of the bisection method, presented in concise steps:

• Choose an initial interval wisely to minimize the number of iterations.
• Handle floating-point limitations by accounting for precision errors in calculations.
• Implement a stopping criterion based on the desired level of accuracy.
• Take advantage of function properties, such as monotonicity, to refine the interval selection.
• Explore modifications and extensions of the method, such as the False Position Method, to improve convergence speed.

Properties of Bisection Method

1. Bracketing Method
   The Bisection Method starts with two values, a and b, such that f(a) and f(b) have opposite signs. This means the function must cross the x-axis between a and b, so there is at least one root in that interval.
2. Continuous Function Required
   The function f(x) must be continuous on the interval [a, b] for the method to work properly.
3. Halves the Interval Each Time
   In every step, the method finds the midpoint of the interval and checks in which half the root lies. This process is repeated, making the interval smaller each time.
4. Slow but Reliable
   The Bisection Method is slower compared to other methods (like Newton-Raphson), but it is very reliable and always converges if the initial conditions are correct.
5. Guaranteed Convergence
   The method always converges to a root as long as the initial interval is correct (i.e., the signs of f(a) and f(b) are different).
6. Accuracy Depends on Number of Iterations
   The more iterations you perform, the closer you get to the actual root. The error reduces by half in each step.

Bisection Method Solved Examples

Example 1: Find the root of the equation
f(x) = x³ - 2x - 5 within the interval [2, 3] using the Bisection Method.

Step 1: Evaluate f(2) and f(3)
f(2) = 2³ - 2×2 - 5 = 8 - 4 - 5 = -1
f(3) = 3³ - 2×3 - 5 = 27 - 6 - 5 = 16

Since f(2) < 0 and f(3) > 0, the function changes sign in the interval [2, 3], so a root exists.

Step 2: Apply the Bisection Method

Iteration 1:
Midpoint c = (2 + 3)/2 = 2.5
f(2.5) = (2.5)³ - 2×2.5 - 5 = 15.625 - 5 - 5 = 5.625
Root lies in [2, 2.5] (because f(2) < 0, f(2.5) > 0)

Iteration 2:
c = (2 + 2.5)/2 = 2.25
f(2.25) = (2.25)³ - 2×2.25 - 5 = 11.39 - 4.5 - 5 = 1.89
Root lies in [2, 2.25]

Iteration 3:
c = (2 + 2.25)/2 = 2.125
f(2.125) = 9.545 - 4.25 - 5 = 0.295

Iteration 4:
c = (2 + 2.125)/2 = 2.0625
f(2.0625) = 8.785 - 4.125 - 5 = -0.34
Root lies in [2.0625, 2.125]

Continue this process...

After several steps, we narrow the interval to around [2.09375, 2.095]
At this point, the root is approximately x ≈ 2.094 (correct to three decimal places).

2) Let's solve the equation within the interval .

Solution

Step 1: Evaluate the function at the interval endpoints:

First, we need to check if there's a root within the given interval. We can do this by evaluating f(x) at the endpoints:

• f(-2) = e(-2) - (-2)3 - 5 = e(-2) + 8 - 5 = e(-2) + 3 = 0.135 + 3 = 3.135
• f(-1) = e(-1) - (-1)3 - 5 = e(-1) + 1 - 5 = e(-1) - 4 = 0.368 - 4 = - 3.632

Step 2: Determine the new interval:

Since f(-2) is positive and f(-1) is negative, and f(x) is a continuous function, the Intermediate Value Theorem guarantees that there is at least one root within the interval [-2, -1].

So, we can bisect the interval and choose the right half for the next iteration:[-1.5, 1]

Repeat steps 1- 5 until the desired level of accuracy is reached. 

Let's assume we obtain a root within a tolerance of 0.001.

By following the steps, after several iterations, we find that the root of the equation 

f(x) = ex − x3 − 5 

within the interval [−2,−1] is approximately -1.689

We hope that the above article is helpful for your understanding and exam preparations. Stay tuned to the Testbook App for more updates on related topics from Mathematics and various such subjects. Also, reach out to the test series available to examine your knowledge regarding several exams.

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