Newton's Method is a powerful calculus technique for approximating solutions to equations that are difficult to solve exactly. It is especially useful for finding roots, which are x-values where f(x) = 0. The method starts with a guess and repeatedly improves it using the slope of the curve.
It matters because many equations in physics, engineering, and data science have no simple algebraic solution.
The key idea is to draw the tangent line to the curve at the current guess, then use where that tangent line crosses the x-axis as the next guess. This process is written as x_(n+1) = x_n - f(x_n)/f'(x_n). When the starting point is close enough to a simple root and the function is well behaved, the estimates often converge very quickly.
However, the method can fail or behave unpredictably if the derivative is zero, the starting guess is poor, or the curve has difficult features near the root.
Key Facts
- Newton's Method formula: x_(n+1) = x_n - f(x_n)/f'(x_n).
- A root is a value r such that f(r) = 0.
- The tangent line at x_n is y = f(x_n) + f'(x_n)(x - x_n).
- The next estimate x_(n+1) is the x-intercept of the tangent line at x_n.
- The method usually converges fastest when x_0 is close to the root and f'(r) is not 0.
- Stop when |x_(n+1) - x_n| is small or when |f(x_n)| is close to 0.
Vocabulary
- Root
- A root is an input value where the function equals zero.
- Iteration
- An iteration is one repeated step of a procedure that uses the current estimate to produce a new estimate.
- Tangent Line
- A tangent line is a line that touches a curve at a point and has the same slope as the curve there.
- Derivative
- A derivative measures the instantaneous rate of change or slope of a function at a point.
- Convergence
- Convergence means the sequence of estimates gets closer and closer to a target value.
Common Mistakes to Avoid
- Using x_(n+1) = x_n + f(x_n)/f'(x_n) is wrong because Newton's Method subtracts the correction term, so the sign changes the direction of the step.
- Dividing by f(x_n) instead of f'(x_n) is wrong because the tangent-line step depends on the slope of the curve, not just the height of the curve.
- Starting too far from the root can give a misleading result because the tangent line may point to a different root, jump away, or enter a cycle.
- Ignoring f'(x_n) = 0 is unsafe because a horizontal tangent makes the Newton formula undefined or causes an extremely large step when the derivative is near zero.
Practice Questions
- 1 Use Newton's Method to approximate a root of f(x) = x^2 - 2 starting with x_0 = 1. Compute x_1, x_2, and x_3.
- 2 For f(x) = x^3 - 5, use x_0 = 2 and the formula x_(n+1) = x_n - f(x_n)/f'(x_n) to compute x_1 and x_2.
- 3 Explain why Newton's Method may fail or converge slowly if the starting guess is placed near a point where the graph has a horizontal tangent.