Calculus How To

Bisection Method: Definition & Example

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The Bisection Method is used to find the root (zero) of a function. It works by successively narrowing down an interval that contains the root. You divide the function in half repeatedly to identify which half contains the root; the process continues until the final interval is very small. The root will be approximately equal to any value within this final interval.

bisection method

The bisection method closes in on the root— a place where the function values is zero (indicated by the red dot).

When working with the bisection method:

  1. Take an interval [a, b] where f(a) and f(b) have opposite signs,
  2. Find the midpoint of [a, b],
  3. Determine whether the root is within [a, (a + b)/2] or [(a + b)/2, b].
  4. Repeat steps 1 through 3 until the interval is small enough.

The Bisection Method & Intermediate Value Theorem

The bisection method is an application of the Intermediate Value Theorem (IVT). As such, it is useful in proving the IVT. The IVT states that suppose you have a segment (between points a and b, inclusive) of a continuous function, and that function crosses a horizontal line. Given these facts, then the intersection of the two lines—point x—must exist.

Example—Solving the Bisection Method

Example Question: Find the 4th approximation of the root of f(x) = x4 – 7 using the bisection method.

Step 1: Find an appropriate starting interval. This is usually an educated guess. The function is continuous, so let’s try (1, 2) as the starting interval.

Step 2: Create a table of values.  In this example we will set up the table for three rows (four approximations).

f(left) f(mid) f(right) New Interval Midpoint Max Error
f(1) = -6 f(1.5) = -2 f(2) = 9 (1.5, 2) 1.75 ±0.25
f(1.5) = -2 f(1.75) = 2.4 f(2) = 9 (1.5, 1.75) 1.625 ±0.125
f(1.5) = -2 f(1.625) = -.03 f(1.75) = 2.4 (1.625, 1.75) 1.6875 ±0.0625

Step 3: Plug the value from Step 2 into the function. The value of the function at x is approximately 1.6875.  The fourth approximation is off by at most ±0.0625.

Notice that each successive approximation builds off of the one preceding it.  At each level in the table we calculate the new interval to be used in the next approximation.

Stephanie Glen. "Bisection Method: Definition & Example" From Calculus for the rest of us!

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