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Using the Mean Value Theorem, we can show the a function is increasing on an interval when its derivative is positive on the interval. Changes in the sign of the derivative detect local extrema. We also can use the second derivative to detect concavity and inflection points. This means that the first and second derivative can be used to classify critical points as local maxima or minima
Citation preview
. . . . . .
Section 4.3Derivatives and the Shapes of
Curves
V63.0121, Calculus I
March 25-26, 2009
.
.Image credit: cobalt123
. . . . . .
Outline
MonotonicityThe Increasing/Decreasing TestFinding intervals of monotonicityThe First Derivative Test
ConcavityDefinitionsTesting for ConcavityThe Second Derivative Test
. . . . . .
The Increasing/Decreasing Test
Theorem (The Increasing/Decreasing Test)If f′ > 0 on (a, b), then f is increasing on (a, b). If f′ < 0 on (a, b), then fis decreasing on (a, b).
Proof.It works the same as the last theorem. Pick two points x and y in(a, b) with x < y. We must show f(x) < f(y). By MVT there exists apoint c in (x, y) such that
f(y) − f(x)y − x
= f′(c) > 0.
Sof(y) − f(x) = f′(c)(y − x) > 0.
. . . . . .
The Increasing/Decreasing Test
Theorem (The Increasing/Decreasing Test)If f′ > 0 on (a, b), then f is increasing on (a, b). If f′ < 0 on (a, b), then fis decreasing on (a, b).
Proof.It works the same as the last theorem. Pick two points x and y in(a, b) with x < y. We must show f(x) < f(y). By MVT there exists apoint c in (x, y) such that
f(y) − f(x)y − x
= f′(c) > 0.
Sof(y) − f(x) = f′(c)(y − x) > 0.
. . . . . .
Finding intervals of monotonicity I
ExampleFind the intervals of monotonicity of f(x) = 2x − 5.
Solutionf′(x) = 2 is always positive, so f is increasing on (−∞,∞).
ExampleDescribe the monotonicity of f(x) = arctan(x).
SolutionSince f′(x) =
11 + x2 is always positive, f(x) is always increasing.
. . . . . .
Finding intervals of monotonicity I
ExampleFind the intervals of monotonicity of f(x) = 2x − 5.
Solutionf′(x) = 2 is always positive, so f is increasing on (−∞,∞).
ExampleDescribe the monotonicity of f(x) = arctan(x).
SolutionSince f′(x) =
11 + x2 is always positive, f(x) is always increasing.
. . . . . .
Finding intervals of monotonicity I
ExampleFind the intervals of monotonicity of f(x) = 2x − 5.
Solutionf′(x) = 2 is always positive, so f is increasing on (−∞,∞).
ExampleDescribe the monotonicity of f(x) = arctan(x).
SolutionSince f′(x) =
11 + x2 is always positive, f(x) is always increasing.
. . . . . .
Finding intervals of monotonicity I
ExampleFind the intervals of monotonicity of f(x) = 2x − 5.
Solutionf′(x) = 2 is always positive, so f is increasing on (−∞,∞).
ExampleDescribe the monotonicity of f(x) = arctan(x).
SolutionSince f′(x) =
11 + x2 is always positive, f(x) is always increasing.
. . . . . .
Finding intervals of monotonicity II
ExampleFind the intervals of monotonicity of f(x) = x2 − 1.
Solution
I f′(x) = 2x, which is positive when x > 0 and negative when x is.I We can draw a number line:
. .f′.− ..0.0 .+
.min
I So f is decreasing on (−∞, 0) and increasing on (0,∞).I In fact we can say f is decreasing on (−∞, 0] and increasing on
[0,∞)
. . . . . .
Finding intervals of monotonicity II
ExampleFind the intervals of monotonicity of f(x) = x2 − 1.
Solution
I f′(x) = 2x, which is positive when x > 0 and negative when x is.
I We can draw a number line:
. .f′.− ..0.0 .+
.min
I So f is decreasing on (−∞, 0) and increasing on (0,∞).I In fact we can say f is decreasing on (−∞, 0] and increasing on
[0,∞)
. . . . . .
Finding intervals of monotonicity II
ExampleFind the intervals of monotonicity of f(x) = x2 − 1.
Solution
I f′(x) = 2x, which is positive when x > 0 and negative when x is.I We can draw a number line:
. .f′.− ..0.0 .+
.min
I So f is decreasing on (−∞, 0) and increasing on (0,∞).I In fact we can say f is decreasing on (−∞, 0] and increasing on
[0,∞)
. . . . . .
Finding intervals of monotonicity II
ExampleFind the intervals of monotonicity of f(x) = x2 − 1.
Solution
I f′(x) = 2x, which is positive when x > 0 and negative when x is.I We can draw a number line:
. .f′
.f
.−.↘
..0.0 .+
.↗
.min
I So f is decreasing on (−∞, 0) and increasing on (0,∞).
I In fact we can say f is decreasing on (−∞, 0] and increasing on[0,∞)
. . . . . .
Finding intervals of monotonicity II
ExampleFind the intervals of monotonicity of f(x) = x2 − 1.
Solution
I f′(x) = 2x, which is positive when x > 0 and negative when x is.I We can draw a number line:
. .f′
.f
.−.↘
..0.0 .+
.↗
.min
I So f is decreasing on (−∞, 0) and increasing on (0,∞).
I In fact we can say f is decreasing on (−∞, 0] and increasing on[0,∞)
. . . . . .
Finding intervals of monotonicity II
ExampleFind the intervals of monotonicity of f(x) = x2 − 1.
Solution
I f′(x) = 2x, which is positive when x > 0 and negative when x is.I We can draw a number line:
. .f′
.f
.−.↘
..0.0 .+
.↗
.min
I So f is decreasing on (−∞, 0) and increasing on (0,∞).I In fact we can say f is decreasing on (−∞, 0] and increasing on
[0,∞)
. . . . . .
Finding intervals of monotonicity IIIExampleFind the intervals of monotonicity of f(x) = x2/3(x + 2).
Solution
f′(x) = 23x−1/3(x + 2) + x2/3 = 1
3x−1/3 (5x + 4)
The critical points are 0 and and −4/5.
. .x−1/3..0
.×.− .+
.5x + 4..−4/5
.0.− .+
.f′(x)
.f(x).
.−4/5
.0 ..0
.×
.+
.↗.−.↘
.+
.↗.max .min
. . . . . .
Finding intervals of monotonicity IIIExampleFind the intervals of monotonicity of f(x) = x2/3(x + 2).
Solution
f′(x) = 23x−1/3(x + 2) + x2/3 = 1
3x−1/3 (5x + 4)
The critical points are 0 and and −4/5.
. .x−1/3..0
.×.− .+
.5x + 4..−4/5
.0.− .+
.f′(x)
.f(x).
.−4/5
.0 ..0
.×
.+
.↗.−.↘
.+
.↗.max .min
. . . . . .
Finding intervals of monotonicity IIIExampleFind the intervals of monotonicity of f(x) = x2/3(x + 2).
Solution
f′(x) = 23x−1/3(x + 2) + x2/3 = 1
3x−1/3 (5x + 4)
The critical points are 0 and and −4/5.
. .x−1/3..0
.×.− .+
.5x + 4..−4/5
.0.− .+
.f′(x)
.f(x).
.−4/5
.0 ..0
.×
.+
.↗.−.↘
.+
.↗.max .min
. . . . . .
Finding intervals of monotonicity IIIExampleFind the intervals of monotonicity of f(x) = x2/3(x + 2).
Solution
f′(x) = 23x−1/3(x + 2) + x2/3 = 1
3x−1/3 (5x + 4)
The critical points are 0 and and −4/5.
. .x−1/3..0
.×.− .+
.5x + 4..−4/5
.0.− .+
.f′(x)
.f(x).
.−4/5
.0 ..0
.×.+
.↗.−.↘
.+
.↗
.max .min
. . . . . .
The First Derivative Test
Theorem (The First Derivative Test)Let f be continuous on [a, b] and c a critical point of f in (a, b).
I If f′(x) > 0 on (a, c) and f′(x) < 0 on (c, b), then c is a localmaximum.
I If f′(x) < 0 on (a, c) and f′(x) > 0 on (c, b), then c is a localminimum.
I If f′(x) has the same sign on (a, c) and (c, b), then c is not a localextremum.
. . . . . .
Finding intervals of monotonicity II
ExampleFind the intervals of monotonicity of f(x) = x2 − 1.
Solution
I f′(x) = 2x, which is positive when x > 0 and negative when x is.I We can draw a number line:
. .f′
.f
.−.↘
..0.0 .+
.↗
.min
I So f is decreasing on (−∞, 0) and increasing on (0,∞).I In fact we can say f is decreasing on (−∞, 0] and increasing on
[0,∞)
. . . . . .
Finding intervals of monotonicity II
ExampleFind the intervals of monotonicity of f(x) = x2 − 1.
Solution
I f′(x) = 2x, which is positive when x > 0 and negative when x is.I We can draw a number line:
. .f′
.f
.−.↘
..0.0 .+
.↗.min
I So f is decreasing on (−∞, 0) and increasing on (0,∞).I In fact we can say f is decreasing on (−∞, 0] and increasing on
[0,∞)
. . . . . .
Finding intervals of monotonicity IIIExampleFind the intervals of monotonicity of f(x) = x2/3(x + 2).
Solution
f′(x) = 23x−1/3(x + 2) + x2/3 = 1
3x−1/3 (5x + 4)
The critical points are 0 and and −4/5.
. .x−1/3..0
.×.− .+
.5x + 4..−4/5
.0.− .+
.f′(x)
.f(x).
.−4/5
.0 ..0
.×.+
.↗.−.↘
.+
.↗
.max .min
. . . . . .
Finding intervals of monotonicity IIIExampleFind the intervals of monotonicity of f(x) = x2/3(x + 2).
Solution
f′(x) = 23x−1/3(x + 2) + x2/3 = 1
3x−1/3 (5x + 4)
The critical points are 0 and and −4/5.
. .x−1/3..0
.×.− .+
.5x + 4..−4/5
.0.− .+
.f′(x)
.f(x).
.−4/5
.0 ..0
.×.+
.↗.−.↘
.+
.↗.max .min
. . . . . .
Outline
MonotonicityThe Increasing/Decreasing TestFinding intervals of monotonicityThe First Derivative Test
ConcavityDefinitionsTesting for ConcavityThe Second Derivative Test
. . . . . .
DefinitionThe graph of f is called concave up on and interval I if it lies aboveall its tangents on I. The graph of f is called concave down on I if itlies below all its tangents on I.
.
concave up
.
concave downWe sometimes say a concave up graph “holds water” and a concavedown graph “spills water”.
. . . . . .
DefinitionA point P on a curve y = f(x) is called an inflection point if f iscontinuous there and the curve changes from concave upward toconcave downward at P (or vice versa).
..concavedown
. concave up
..inflection point
. . . . . .
Theorem (Concavity Test)
I If f′′(x) > 0 for all x in I, then the graph of f is concave upward on II If f′′(x) < 0 for all x in I, then the graph of f is concave downward on I
Proof.Suppose f′′(x) > 0 on I. This means f′ is increasing on I. Let a and xbe in I. The tangent line through (a, f(a)) is the graph of
L(x) = f(a) + f′(a)(x − a)
By MVT, there exists a b between a and x withf(x) − f(a)
x − a= f′(b). So
f(x) = f(a) + f′(b)(x − a) ≥ f(a) + f′(a)(x − a) = L(x)
. . . . . .
Theorem (Concavity Test)
I If f′′(x) > 0 for all x in I, then the graph of f is concave upward on II If f′′(x) < 0 for all x in I, then the graph of f is concave downward on I
Proof.Suppose f′′(x) > 0 on I. This means f′ is increasing on I. Let a and xbe in I. The tangent line through (a, f(a)) is the graph of
L(x) = f(a) + f′(a)(x − a)
By MVT, there exists a b between a and x withf(x) − f(a)
x − a= f′(b). So
f(x) = f(a) + f′(b)(x − a) ≥ f(a) + f′(a)(x − a) = L(x)
. . . . . .
ExampleFind the intervals of concavity for the graph of f(x) = x3 + x2.
Solution
I We have f′(x) = 3x2 + 2x, so f′′(x) = 6x + 2.I This is negative when x < −1/3, positive when x > −1/3, and 0 when
x = −1/3
I So f is concave down on (−∞,−1/3), concave up on (1/3,∞), andhas an inflection point at (−1/3, 2/27)
. . . . . .
ExampleFind the intervals of concavity for the graph of f(x) = x3 + x2.
Solution
I We have f′(x) = 3x2 + 2x, so f′′(x) = 6x + 2.
I This is negative when x < −1/3, positive when x > −1/3, and 0 whenx = −1/3
I So f is concave down on (−∞,−1/3), concave up on (1/3,∞), andhas an inflection point at (−1/3, 2/27)
. . . . . .
ExampleFind the intervals of concavity for the graph of f(x) = x3 + x2.
Solution
I We have f′(x) = 3x2 + 2x, so f′′(x) = 6x + 2.I This is negative when x < −1/3, positive when x > −1/3, and 0 when
x = −1/3
I So f is concave down on (−∞,−1/3), concave up on (1/3,∞), andhas an inflection point at (−1/3, 2/27)
. . . . . .
ExampleFind the intervals of concavity for the graph of f(x) = x3 + x2.
Solution
I We have f′(x) = 3x2 + 2x, so f′′(x) = 6x + 2.I This is negative when x < −1/3, positive when x > −1/3, and 0 when
x = −1/3
I So f is concave down on (−∞,−1/3), concave up on (1/3,∞), andhas an inflection point at (−1/3, 2/27)
. . . . . .
ExampleFind the intervals of concavity of the graph of f(x) = x2/3(x + 2).
Solution
I f′′(x) =109
x−1/3 − 49
x−4/3 =29
x−4/3(5x − 2)
I x−4/3 is always positive, so the concavity is determined by the 5x − 2factor
I So f is concave down on (−∞, 2/5), concave up on (2/5,∞), and hasan inflection point when x = 2/5
. . . . . .
ExampleFind the intervals of concavity of the graph of f(x) = x2/3(x + 2).
Solution
I f′′(x) =109
x−1/3 − 49
x−4/3 =29
x−4/3(5x − 2)
I x−4/3 is always positive, so the concavity is determined by the 5x − 2factor
I So f is concave down on (−∞, 2/5), concave up on (2/5,∞), and hasan inflection point when x = 2/5
. . . . . .
ExampleFind the intervals of concavity of the graph of f(x) = x2/3(x + 2).
Solution
I f′′(x) =109
x−1/3 − 49
x−4/3 =29
x−4/3(5x − 2)
I x−4/3 is always positive, so the concavity is determined by the 5x − 2factor
I So f is concave down on (−∞, 2/5), concave up on (2/5,∞), and hasan inflection point when x = 2/5
. . . . . .
ExampleFind the intervals of concavity of the graph of f(x) = x2/3(x + 2).
Solution
I f′′(x) =109
x−1/3 − 49
x−4/3 =29
x−4/3(5x − 2)
I x−4/3 is always positive, so the concavity is determined by the 5x − 2factor
I So f is concave down on (−∞, 2/5), concave up on (2/5,∞), and hasan inflection point when x = 2/5
. . . . . .
The Second Derivative Test
Theorem (The Second Derivative Test)Let f, f′, and f′′ be continuous on [a, b]. Let c be be a point in (a, b) withf′(c) = 0.
I If f′′(c) < 0, then f(c) is a local maximum.I If f′′(c) > 0, then f(c) is a local minimum.
If f′′(c) = 0, the second derivative test is inconclusive (this does notmean c is neither; we just don’t know yet).
. . . . . .
ExampleFind the local extrema of f(x) = x3 + x2.
Solution
I f′(x) = 3x2 + 2x = x(3x + 2) is 0 when x = 0 or x = −2/3.I Remember f′′(x) = 6x + 2I Since f′′(−2/3) = −2 < 0, −2/3 is a local maximum.I Since f′′(0) = 2 > 0, 0 is a local minimum.
. . . . . .
ExampleFind the local extrema of f(x) = x3 + x2.
Solution
I f′(x) = 3x2 + 2x = x(3x + 2) is 0 when x = 0 or x = −2/3.
I Remember f′′(x) = 6x + 2I Since f′′(−2/3) = −2 < 0, −2/3 is a local maximum.I Since f′′(0) = 2 > 0, 0 is a local minimum.
. . . . . .
ExampleFind the local extrema of f(x) = x3 + x2.
Solution
I f′(x) = 3x2 + 2x = x(3x + 2) is 0 when x = 0 or x = −2/3.I Remember f′′(x) = 6x + 2
I Since f′′(−2/3) = −2 < 0, −2/3 is a local maximum.I Since f′′(0) = 2 > 0, 0 is a local minimum.
. . . . . .
ExampleFind the local extrema of f(x) = x3 + x2.
Solution
I f′(x) = 3x2 + 2x = x(3x + 2) is 0 when x = 0 or x = −2/3.I Remember f′′(x) = 6x + 2I Since f′′(−2/3) = −2 < 0, −2/3 is a local maximum.
I Since f′′(0) = 2 > 0, 0 is a local minimum.
. . . . . .
ExampleFind the local extrema of f(x) = x3 + x2.
Solution
I f′(x) = 3x2 + 2x = x(3x + 2) is 0 when x = 0 or x = −2/3.I Remember f′′(x) = 6x + 2I Since f′′(−2/3) = −2 < 0, −2/3 is a local maximum.I Since f′′(0) = 2 > 0, 0 is a local minimum.
. . . . . .
ExampleFind the local extrema of f(x) = x2/3(x + 2)
Solution
I Remember f′(x) =13
x−1/3(5x + 4) which is zero when x = −4/5
I Remember f′′(x) =109
x−4/3(5x − 2), which is negative when
x = −4/5
I So x = −4/5 is a local maximum.I Notice the Second Derivative Test doesn’t catch the local minimum
x = 0 since f is not differentiable there.
. . . . . .
ExampleFind the local extrema of f(x) = x2/3(x + 2)
Solution
I Remember f′(x) =13
x−1/3(5x + 4) which is zero when x = −4/5
I Remember f′′(x) =109
x−4/3(5x − 2), which is negative when
x = −4/5
I So x = −4/5 is a local maximum.I Notice the Second Derivative Test doesn’t catch the local minimum
x = 0 since f is not differentiable there.
. . . . . .
ExampleFind the local extrema of f(x) = x2/3(x + 2)
Solution
I Remember f′(x) =13
x−1/3(5x + 4) which is zero when x = −4/5
I Remember f′′(x) =109
x−4/3(5x − 2), which is negative when
x = −4/5
I So x = −4/5 is a local maximum.I Notice the Second Derivative Test doesn’t catch the local minimum
x = 0 since f is not differentiable there.
. . . . . .
ExampleFind the local extrema of f(x) = x2/3(x + 2)
Solution
I Remember f′(x) =13
x−1/3(5x + 4) which is zero when x = −4/5
I Remember f′′(x) =109
x−4/3(5x − 2), which is negative when
x = −4/5
I So x = −4/5 is a local maximum.
I Notice the Second Derivative Test doesn’t catch the local minimumx = 0 since f is not differentiable there.
. . . . . .
ExampleFind the local extrema of f(x) = x2/3(x + 2)
Solution
I Remember f′(x) =13
x−1/3(5x + 4) which is zero when x = −4/5
I Remember f′′(x) =109
x−4/3(5x − 2), which is negative when
x = −4/5
I So x = −4/5 is a local maximum.I Notice the Second Derivative Test doesn’t catch the local minimum
x = 0 since f is not differentiable there.
. . . . . .
Graph
Graph of f(x) = x2/3(x + 2):
. .x
.y
..(−4/5, 1.03413)
..(0, 0)
..(2/5, 1.30292)
..(−2, 0)
. . . . . .
When the second derivative is zero
I At inflection points c, if f′ is differentiable at c, then f′′(c) = 0I Is it necessarily true, though?
Consider these examples:
f(x) = x4 g(x) = −x4 h(x) = x3
All of them have f′′(0) = 0. But the first has a local min at 0, thesecond has a local max at 0, and the third has an inflection point at 0.This is why we say 2DT has nothing to say when f′′(c) = 0.
. . . . . .
When the second derivative is zero
I At inflection points c, if f′ is differentiable at c, then f′′(c) = 0I Is it necessarily true, though?
Consider these examples:
f(x) = x4 g(x) = −x4 h(x) = x3
All of them have f′′(0) = 0. But the first has a local min at 0, thesecond has a local max at 0, and the third has an inflection point at 0.This is why we say 2DT has nothing to say when f′′(c) = 0.
. . . . . .
When the second derivative is zero
I At inflection points c, if f′ is differentiable at c, then f′′(c) = 0I Is it necessarily true, though?
Consider these examples:
f(x) = x4 g(x) = −x4 h(x) = x3
All of them have f′′(0) = 0. But the first has a local min at 0, thesecond has a local max at 0, and the third has an inflection point at 0.This is why we say 2DT has nothing to say when f′′(c) = 0.
. . . . . .
Summary
I Concepts: Mean Value Theorem, monotonicity, concavityI Facts: derivatives can detect monotonicity and concavityI Techniques for drawing curves: the Increasing/Decreasing Test
and the Concavity TestI Techniques for finding extrema: the First Derivative Test and the
Second Derivative Test
Next week: Graphing functions
. . . . . .
Summary
I Concepts: Mean Value Theorem, monotonicity, concavityI Facts: derivatives can detect monotonicity and concavityI Techniques for drawing curves: the Increasing/Decreasing Test
and the Concavity TestI Techniques for finding extrema: the First Derivative Test and the
Second Derivative Test
Next week: Graphing functions