Lecture 9 Notes: Curve Sketching

18.01 Calculus Jason Starr Fall 2005 Lecture 9. September 29, 2005 Homework. Problem Set 2 all of Part I and Part II. Practice Problems. Course Reader: 2B­1, 2B­2, 2B­4, 2B­5. 1. Application of the Mean Value Theorem. A real­world application of the Mean Value Theorem is error analysis. A device accepts an input signal x and returns an output signal y. If the input signal is always in the range −1/2 ≤ x ≤ 1/2 and if the output signal is, y = f (x) = 1 , 1 + x + x2 + x3 what precision of the input signal x is required to get a precision of ±10−3 for the output signal? If the ideal input signal is x = a, and if the precision is ±h, then the actual input signal is in the range a − h ≤ x ≤ a + h. The precision of the output signal is |f (x) − f (a)|. By the Mean Value Theorem, f (x) − f (a) = f � (c), x−a for some c between a and x. The derivative f � (x) is, f � (x) = −(3x2 + 2x + 1) . (1 + x + x2 + x3 )2 For −1/2 ≤ x ≤ 1/2, this is bounded by, 3(1/2)2 + 2(1/2) + 1 |f (x)| ≤ = 7.04. [1 + (−1/2) + (−1/2)2 + (−1/2)3 ]2 � Thus the Mean Value Theorem gives, |f (x) − f (a)| = |f � (c)||x − a| ≤ 7.04|x − a| ≤ 7.04h. Therefore a precision for the input signal of, h = 10−3 /7.04 ≈ 10−4 guarantees a precision of 10−3 for the output signal. 2. First derivative test. A function f (x) is increasing, respectively decreasing, if f (a) is less than f (b), resp. greater than f (b), whenever a is less than b. In symbols, f is increasing, respectively decreasing, if f (a) < f (b) whenever a < b, resp. f (a) > f (b) whenever a < b. 18.01 Calculus Jason Starr Fall 2005 If f (a) is less than or equal to f (b), resp. greater than or equal to f (b), whenever a is less than b, then f (x) is non­decreasing, resp. non­increasing. If f (x) is increasing, the graph rises to the right. If f (x) is decreasing, the graph rises to the left. If f � (a) is positive, the First Derivative Test guarantees that f (x) is increasing for all x sufficiently close to a. If f � (a) is negative, the First Derivative Test guarantees that f (x) is decreasing for all x sufficiently close to a. Example. For the function y = x3 + x2 − x − 1, determine where y is increasing and where y is decreasing. The derivative is, y � = 3x2 + 2x − 1 = (3x − 1)(x + 1). Thus the derivative of y changes sign only at the points x = −1 and x = 1/3. By testing random elements, y � is positive for x > 1/3, it is negative for −1 < x < 1/3, and it is positive for x < −1. Therefore, by the First Derivative Test, y is increasing for x < −1, y is decreasing for −1 < x < 1/3, and y is increasing for x > 1/3. 3. Extremal points. If f (x) ≤ f (a) for all x near a, then x is a local maximum. If f (x) ≥ f (a) for all x near a, then x is a local minimum. Because of the First Derivative Test, if f � (a) > 0 and f is defined to the right of a, the graph of f rises to the right of a. Thus a is not a local maximum. Similarly, if f � (a) < 0 and f is defined to the left of a, the graph of f rises to the left of a. Thus a is not a local maximum. In particular, if f is defined to both the right and left of a, if f � (a) is defined, and if a is a local maximum, then f � (a) equals 0. Similarly, if f is defined to both the right and left of a, if f � (a) is defined, and if a is a local minimum, then f � (a) equals 0. A point a where f � (a) is defined and equals 0 is a critical point. By the last paragraph, if x = a is a local maximum of f , respectively a local minimum of f , then one of the following holds. (i) The function f (x) is discontinuous at a. (ii) The function f (x) is continuous at a, but f � (a) is not defined. (iii) The point a is a left endpoint of the interval where f is defined, and f � (a) ≤ 0, resp. f � (a) ≥ 0. (iv) The point a is a right endpoint of the interval where f is defined, and f � (a) ≥ 0, rexp. f � (a) ≤ 0. (v) The function f is defined to the left and right of a, and f � (a) equals 0. In other words, a is a critical point of f . Example. For the function y = x3 + x2 − x − 1, the critical points are x = −1 and x = 1/3. By examining where y is increasing and decreasing, x = −1 is a local maximum and x = 1/3 is a local minimum. The plurals of “maximum” and “minimum” are “maxima” and “minima”. Together, local maxima and local minima are called extremal points, or extrema. These are points where f takes on an 18.01 Calculus Jason Starr Fall 2005 extreme value, either positive or negative. A point where f achieves its maximum value among all points where f is defined is a global maximum or absolute maximum. A point where f achieves its minimum value among all points where f is defined is a global minimum or absolute minimum. 4. Concavity and the Second Derivative Test. For a differentiable function f , every “interior” extremal point is a critical point of f . But not every critical point of f is an extremal point. Example. The function f (x) = x3 has a critical point at x = 0. But f (x) is everywhere increasing, thus x = 0 is not an extremal point of f . When is a critical point an extremal point? When is it a local maximum? When is it a local minimum? This is closely related to the concavity of f . A function f (x) is concave up, respectively concave down, if no secant line segment to f (x) crosses below the graph of f , resp. above the graph of f . In symbols, f is concave up, resp. concave down, if (f (c) − f (a))/(c − a) ≤ (f (b) − f (a))/(b − a) whenever a < c < b, resp. (f (c) − f (a))/(c − a) ≥ (f (b) − f (a))/(b − a) whenever a < c < b. For a differentiable function f , this equation is close to, f � (c) ≤ f � (b) whenever a < c < b, resp. f � (c) ≥ f � (b) whenever a > c > b. This precisely says that f � is non­decreasing, resp. f � is non­increasing. If f � is non­decreasing, resp. non­increasing, then f is concave up, resp. concave down. Applying the First Derivative Test to determine when f � is increasing, resp. decreasing, gives the Second Derivative Test : If f �� (a) > 0, then f is concave up near x = a; if f �� (a) < 0 then f is concave down near x = a. If f is concave up near a critical point, the critical point is a local minimum. If f is concave down near a critical point, the critical point is a local maximum. Combined with the Second Derivative Test, this gives a test for when a critical point is a local maximum or local minimum: If f � (a) equals 0 and f �� (a) < 0, then x = a is a local maximum. If f � (a) equals 0 and f �� (a) > 0, then x = a is a local minimum. Example. For y = x3 + x2 − x − 1, the second derivative is y �� = 6x + 2. Since y �� (−1) = −4 is negative, the critical point x = −1 is a local maximum. Since y �� (1/3) = 4 is positive, x = 1/3 is a local minimum. 5. Inflection points. If f is differentiable, but for every neighborhood of a, f is neither concave up nor concave down on the entire neighborhood, then a is an inflection point. If f �� (a) is defined, the Second Derivative Test says that f �� (a) must equal 0. Except in pathological cases, an inflection point is a point where f is concave up to one side of f , and concave down to the other side of f . Example. For y = x3 + x2 − x − 1, the second derivative y �� = 6x + 2 is negative for x < −1/3 and is positive for x > 1/3. By the Second Derivative Test, y is concave down for x < −1/3 and y is concave up for x > −1/3. Therefore x = −1/3 is an inflection point for y.