1. AP CALCULUS AB Chapter 4: Applications of Derivatives Section 4.5:
Linearization and Newton’s Method

2. What you’ll learn about
Linear Approximation
Newton’s Method
Differentials
Estimating Change with Differentials
Absolute, Relative, and Percent Change
Sensitivity to Change
…and why
Engineering and science depend on approximation in most practical applications; it is important to understand how approximation techniques work.

3. Linear Approximation Any differentiable curve is “Locally Linear”
if you zoom in enough times.
Do Exploration 1: Appreciating Local Linearity (p 233)
A fancy name for the equation of the tangent line at a is
“The linearization of f at a”
y – f(a) = f’(a)(x – a)

4. Definition of Linearization

5. Just Math Tutoring You Tube
What is Linearization?
Just math tutoring
Finding the Linearization at a point
Followed by
25) Linear Approximation
10 minutes total time needed – Watch if you miss class this day or do not understand!

6. Example 1 Finding a Linearization
Find the linearization of at x = 0 (center of approximation) and use it to approximate without a calculator.
Then use a calculator to determine the accuracy of the approximation.
Point of tangency f ‘(0) =
L(x) = Equation of the tangent line:
Evaluate L(.02)
Calculator approximation?
Approximation error:

7. You try: Find linearization L(x) of f(x) at x = a when. and a = 2
You try: Find linearization L(x) of f(x) at x = a when and a = 2. How accurate is the approximation L(a + 0.1) ≈ f(a + 0.1)
Point of tangency f(2) = f ’(2)
Tangent Line equation: L(x)
Evaluate |L(2.1) – f(2.1)|
Approximation error:

8. Point of tangency f (π/2) f ’(π/2) Tangent Line equation: L(x)
Example 2: Find the linearization of f(x) = cos x at x = π/2 and use it to approximate cos 1.75 without a calculator. Then use a calculator to determine the accuracy of the approximation.
Point of tangency f (π/2) f ’(π/2)
Tangent Line equation:
L(x)
Evaluate |L(1.75) – cos 1.75 by calculator |
Approximation error:

9. Example Finding a Linearization

10. Summary
Every function is “locally linear” about a point x = a. If you evaluate the tangent line at x = a for points close to a, you will have a close approximation to the function’s actual value.

11. Steps
Using f(x), find the equation of a tangent line at some point (a, f(a)).
Find f(a) by plugging a into f(x).
Find the slope from f’(a).
L(x) = f(a) + f’(a) (x - a).
2) Evaluate L(x) for any x near a to get a close approximation of f(x) for points near a.

12. Example 3: Approximating Binomial Powers using the general formula
Use the formula to find polynomials that will approximate the following functions for values of x close to zero.
b) c) d)
How?
Rewrite expression as (1 + x) k,
Identify coefficients of x and k.
Find L(x) = 1 + kx for each expression.

13. Example 4: Use linearizations to approximate roots. Find a) and b)
Identify function: f(x) =
Let a be the perfect square closest to Find L(x) at x = a.
Use L(x) to estimate
Error?
You try b.

14. Differentials
(With dx as in independent variable and dy a dependent variable that depends on both x and dx.)
Although Liebniz did most of his calculus using dy and dx as separable entities, he never quite settled the issue of what they were. To him, they were “infinitesimals” – nonzero numbers, but infinitesimally small. There was much debate about whether such things could exist in mathematics, but luckily for the early development of calculus it did not matter: thanks to the Chain Rule, dy/dx behaved like a quotient whether it was one or not.

15. Example Finding the Differential dy

16. Example 6 Find the differential dy and evaluate dy for the given values of x and dx. How? Find f ’(x), multiply both sides by dx, evaluate for given values.
a) y = x5 + 37x b) y = sin 3x c) x + y = xy
x=1, dx = x=π, dx = x=2, dx = 0.05
You try:

17. More Notation…

18. Example 7 Finding Differentials of functions
Example 7 Finding Differentials of functions. Find dy/dx and multiply both sides by dx.
d (tan (2x)) b)
You try: d(e5x + x5)

19. Estimating Change with Differentials
Suppose we know the value of a differentiable function f(x) at a point a and we want to predict how much this value will change if we move to a nearby point (a + dx).
If dx is small, f and its linearization L at “a” will change by nearly the same amount.
Since the values of L are simple to calculate, calculating the change in L offers a practical way to estimate the change in f.

20. Differential Estimate of Change

21. Estimating Change with Differentials

22. Example Estimating Change with Differentials

23. Area formula for a circle: A = True change: f(10.1) – f(10) =
Example 8 The radius r of a circle increases from a = 10 to 10.1 m. Use dA to estimate the increase in the circle’s area A. Compare this estimate with the true change ∆A, and find the approximation error.
Area formula for a circle: A =
True change: f(10.1) – f(10) =
Estimated change: dA/dr =
dA =
Approximation error: |∆A – dA| =
You try: f(x) = x3 - x, a = 1, dx = 0.1

24. In Review:
The linear approximation of a differentiable function at c is
because, from the slope of the tangent line

25. In Review Definition of Differentials:
is a differentiable function in an open interval containing x.
The differential of x is any non-zero real number.
The differential of y is

26. Summary
Linearization: The equation of a tangent line to f at a point a will give a good approximation of the value of a function f at a.
The Linearization of (1 + x)k = 1 + kx
Newton’s Method is used to find the roots of a function by using successive tangent line approximations, moving closer and closer to the roots of f if you start with a reasonable value of a.
Differentials: Differentials simply estimate the change in y as it relates to the change in x for given values of x. We learned how to estimate with linearizations, differentials are simply a more efficient method of finding change.

27. Just Math Tutoring – Newton’s Method (7:29 minutes)
FYI – not tested Newton’s Method for approximating a zero of a function
Approximate the zero of a function by finding the zeros of linearizations converging to an accurate approximation.
Just Math Tutoring – Newton’s Method
(7:29 minutes)

28. Procedure for Newton’s Method

29. Procedure for Newton’s Method

30. Using Newton’s Method