Lesson 14: Derivatives of Exponential and Logarithmic Functions (Section 041 handout)
Derivatives of Logarithmic Functions
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Transcript of Derivatives of Logarithmic Functions
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Derivatives of Logarithmic Functions
Objective: Obtain derivative formulas for logs.
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Review Laws of Logs
• Algebraic Properties of Logarithms
1. Product Property
2. Quotient Property
3. Power Property
4. Change of base
caac bbb loglog)(log
caca bbb loglog)/(log
ara br
b log)(log
bc
bccb log
loglnlnlog
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Review Laws of Logs
• Algebraic Properties of Logarithms
• Remember that means .xy blog xb y
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Review Laws of Logs
• Algebraic Properties of Logarithms
• Remember that means .• Logarithmic and exponential functions are inverse
functions.
xy blog xb y
xby xy blog
xb bx logxbbx log
ybx yx blog
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Derivatives of Logs
• We will start this definition with another way to express e. In chapter 2, we defined e as:
• Now, we will look at e as:
• We make the substitution v = 1/x, and we know that as
ex
x
x
11lim
ev v
v
/1
0)1(lim
x 0v
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Defintion
hxhxx
dxd
h
ln)ln(lim][ln0
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Defintion
hxhxx
dxd
h
ln)ln(lim][ln0
x
hxhhln1lim
0
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Defintion
hxhxx
dxd
h
ln)ln(lim][ln0
x
hxhhln1lim
0
xh
hh1ln1lim
0
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Definition
• We will now let v=h/x, so h = vx
xh
hh1ln1lim
0
)1ln(1lim0
vvxv
)1ln(1lim10
vvx v
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Definition
• Finally
v
vv
x/1
0)1ln(lim1
])1(limln[1 /1
0
v
vv
x
)1ln(1lim10
vvx v
exln1
xx
dxd 1][ln
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Defintion
• Now we will look at the derivative of a log with any base.
][log xdxd
b
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Defintion
• Now we will look at the derivative of a log with any base.
• We will use the change of base formula to rewrite this as
][log xdxd
b
bx
dxdlnln
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Defintion
• Now we will look at the derivative of a log with any base.
• We will use the change of base formula to rewrite this as
][log xdxd
b
bxx
dxd
bbx
dxd
ln1][ln
ln1
lnln
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Definition
• In summary:
xx
dxd 1][ln
bxx
dxd
b ln1][log
dxdu
uu
dxd
1][ln
dxdu
buu
dxd
b ln1][log
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Example 1
• The figure below shows the graph of y = lnx and its tangent lines at x = ½, 1, 3, and 5. Find the slopes of the tangent lines.
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Example 1
• The figure below shows the graph of y = lnx and its tangent lines at x = ½, 1, 3, and 5. Find the slopes of the tangent lines.
• Since the derivative of y = lnx is dy/dx = 1/x, the slopes of the tangent lines are: 2, 1, 1/3, 1/5.
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Example 1
• Does the graph of y = lnx have any horizontal tangents?
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Example 1
• Does the graph of y = lnx have any horizontal tangents?
• The answer is no. 1/x (the derivative) will never equal zero, so there are no horizontal tangent lines.
• As the value of x approaches infinity, the slope of the tangent line does approach 0, but never gets there.
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Example 2
• Find )]1[ln( 2 xdxd
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Example 2
• Find
• We will use a u-substitution and let
)]1[ln( 2 xdxd
12 xu
xdxdu 2
uu
dxd 1][ln
12)]1[ln( 2
2
xxx
dxd
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Example 3
• Find
xxx
dxd
1sinln
2
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Example 3
• Find
• We will use our rules of logs to make this a much easier problem.
xxx
dxd
1sinln
2
)1ln(21)ln(sinln2
1sinln
2
xxxdxd
xxx
dxd
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Example 3
• Now, we solve.
)1ln(
21)ln(sinln2 xxx
dxd
)1(21
sincos2
xxx
x
xx
x 221cot2
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Absolute Value
• Lets look at |]|[ln xdxd
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Absolute Value
• Lets look at
• If x > 0, |x| = x, so we have
|]|[ln xdxd
xx
dxdx
dxd 1][ln|]|[ln
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Absolute Value
• Lets look at
• If x > 0, |x| = x, so we have
• If x < 0, |x|= -x, so we have
|]|[ln xdxd
xx
dxdx
dxd 1][ln|]|[ln
xxx
dxdx
dxd 11][ln|]|[ln
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Absolute Value
• Lets look at
• If x > 0, |x| = x, so we have
• If x < 0, |x|= -x, so we have
• So we can say that
|]|[ln xdxd
xx
dxdx
dxd 1][ln|]|[ln
xxx
dxdx
dxd 11][ln|]|[ln
xx
dxd 1|]|[ln
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Logarithmic Differentiation
• This is another method that makes finding the derivative of complicated problems much easier.
• Find the derivative of
42
32
)1(147
xxxy
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Logarithmic Differentiation
• Find the derivative of
• First, take the natural log of both sides and treat it like example 3.
42
32
)1(147
xxxy
)1ln(4)147ln(31ln2ln 2xxxy
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Logarithmic Differentiation
• Find the derivative of
• First, take the natural log of both sides and treat it like example 3.
42
32
)1(147
xxxy
)1ln(4)147ln(31ln2ln 2xxxy
218
)147(3721
xx
xxdxdy
y
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Logarithmic Differentiation
• Find the derivative of 42
32
)1(147
xxxy
218
)147(3721
xx
xxdxdy
y
42
32
2 )1(147
18
6312
xxx
xx
xxdxdy
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Homework
• Section 3.2• 1-29 odd• 35, 37