question if f is differentiable, find the limit sol
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QuestionIf f is differentiable, find the limit
Sol
.)()3(
lim22
0 h
hxfhxfh
).()(8)]()([)(4
)]}()3([)()3(
{lim
)()3(lim
0
22
0
xfxfxfxfxf
hxfhxfh
hxfhxfh
hxfhxf
h
h
Question Find the limit:
(1) (2)
Sol. (1)
(2)
0
sin 4lim
sin 6x
x
x 0
tan 6lim
sin 2x
x
x
0 0
sin 4 sin 4 6 4 2 2lim lim 1 1
sin 6 4 sin 6 6 3 3x x
x x x
x x x
0 0
tan 6 sin 6 2 1 6lim lim 1 1 1 3 3
sin 2 6 sin 2 cos 6 2x x
x x x
x x x x
The Chain RuleTheorem If u=g(x) is differentiable at x=a and y=f(u) is
differentiable at u=g(a). Then y=f(g(x)) is differentiable at
x=a, and
or,
[ ( ( ))] ( ) ( ) ( ( )) ( ),f g x f u g x f g x g x
dy dy du
dx du dx
Derivative of power functionsEx. Differentiate
Sol. Since can be written as the composition
by the chain rule, we have
.y x
ln xy e
ln 1( ) ( ) u a xy f u g x e e x xx x x
( ) , ( ) ln ,uf u e u g x x
Derivative of exponential functions
Ex. Differentiate
Sol. Since can be written into
by the chain rule, we have
xy a
lnx x ay a e
ln( ) ( ) ln ln lnu x a xy f u g x e a e a a a
( ) , ( ) (ln ) ,uf u e u g x a x
ExampleEx. Differentiate
Sol. Let then
By the chain rule, we have
2( ) 1F x x
2( ) , ( ) 1,f u u u g x x ( ) ( ( )).F x f g x
1/ 2
2
1( ) ( ) ( ) 2
2 1
x xF x f u g x u x
u x
The power rule The power rule combined with the chain rule
Ex. Find the derivative of
Sol.
1[ ( )] ( ) ( )n nf x nf x f x 9
2( )
2 1
tg t
t
8
2 2( ) 9
2 1 2 1
t tg t
t t
8 8
2 10
2 (2 1) 1 ( 2) 2 45( 2)9
2 1 (2 1) (2 1)
t t t t
t t t
The chain ruleIf y=f(u), u=g(v) and v=h(x) are all differentiable, then
y=f(g(h(x))) is differentiable and
or,
[ ( ( ( )))] ( ) ( ) ( ) ( ( ( ))) ( ( )) ( )f g h x f u g v h x f g h x g h x h x
dy dy du dv
dx du dv dx
Example Ex. Differentiate
Sol.
( ) sin(cos(tan ))f x x
2( ) cos(cos(tan )) [ sin(tan )] secf x x x x
2cos(cos(tan )) sin(tan ) secx x x
Logarithmic differentiationEx. Find the derivative of
Sol. Not a power function, not an exponential function
Since by product rule and chain rule,
The method used here is called logarithmic differentiation
2tan ln(1 ) 2 22
2 tan 2 22
2[sec ln(1 ) tan ]
12 tan
(1 ) [sec ln(1 ) ].1
x x
x
xy e x x x
xx x
x x xx
2 tan(1 ) .xy x
2tan ln(1 ) ,x xy e
Logarithmic differentiationIn general, to differentiate we can take logarithm
first to get then differentiating both sides
Question: Find the derivative of
Sol.
( ) .1
xx
f xx
( )( ) ,g xy f xln ( ) ln ( ),y g x f x
( )( ) ( )( ) ln ( ) ( ) ( ) [ ( ) ln ( ) ( ) ].
( ) ( )g xy f x f x
g x f x g x y f x g x f x g xy f x f x
1( ) ln .
1 1 1
xx x
f xx x x
Question Differentiate
Sol.
2 3
23
( 1) 3 2.
( 3)
x xy
x
2
5 23
( 1)(15 74 31).
3 ( 3) (3 2)
x x xy
x x
Implicit differentiation Materials in textbook: page 227-233
Outline Derivative of implicit functions Derivative of inverse trigonometric functions
Expressions of functions Explicit expression: y can be explicitly expressed in term
of x, for example,
Implicit expression: x and y related by an equation, and can not solve y in terms of x explicitly, for example,
3 1y x
4 3 arcsin .xyx y e x
Implicit differentiationEx. Find if
Sol. Differentiating both sides with respect to x, regarding y
as a function of x, and using the chain rule, we get
Solving the equation for we obtain
Ex. Find an equation of the tangent line to the curve
at the origin.
Sol. is the slope
y 3 3 6 .x y xy
2 23 3 6 6 .x y y y xy ,y
2
2
2.
2
y xy
y x
4 3 2 3 x y x y4 2 3 3 2
3 4 2 3 (0)3
x y y x y y y
Example Suppose y=f(x) is defined implicitly by
(1) Find (2) Let find
Sol. (1)
(2)
ln 1. xy y( )( ) (ln ) , f xg x f x e( )f x (1).g
21 ( )( ) ( ) ( ) 0 ( )
( ) 1 ( )
f xf x xf x f x f x
f x xf x
( ) ( )
(1) (1)
1( ) (ln ) (ln ) ( )
(1) (0) (0) (1).
f x f x
f f
g x e f x f x e f xx
g e f f e f2 1
(0) , (1) 1, (0) , (1) .2
f e f f e f2 31
(1) .2
g e e
Homework 5 Section 3.1: 45, 56, 57
Section 3.2: 10, 21, 42
Section 3.4: 11, 16, 38, 39, 42
Section 3.5: 20, 28, 40
Section 3.6: 10, 18
Derivative of arcsine function Ex. Find the derivative of
Analysis. means
differentiating will give
Sol. Differentiating implicitly with respect to x,
we obtain so
sinx yarcsiny x( )y x
arcsin .y x
2
1(arcsin )
1x
x
sinx ysinx y
1 cos y y
2
1 1
cos 1y
y x
Derivative of inverse functions If x=f(y) is differentiable and then the inverse
function is differentiable and
or,
Proof.
1 1[ ( )] ,
( )f x
f y
( ) 0,f y 1( )y f x
1.
dydxdxdy
1( ) 1 ( ) ( ) ( )
( )x f y f y y x y x
f y
Example Similarly,
22
2
1 1(arccos ) , (arctan ) ,
111
( cot ) .1
x xxx
arc xx
Higher derivatives The derivative of is called the second derivative of
f and denoted by or
Recursively, we can define the third derivative and generally the nth derivative
Interpretation: for example, if s(t) is displacement, then is velocity, is acceleration and is jerk.
( )f x( ) , f f
22
2( ) , ( )
d dy d yD f x
dx dx dx
( ) , f f
( 1) ( )( ) ( ). n
n n nn
d yf f D f x
dx( )s t
( )s t( )s t
ExampleIf
then
3 26 5 3y x x x
23 12 5y x x
6 12y x
6y( ) 0 ( 4)ny n
ExampleFind if
Sol.
At x=0, y=1, and
thus
. ye xy e(0)y
2
0
( ) (1 )
( )
yy
y y
y
ye y y xy y
x e
y x e y e yy
x e
0
1(0) ,
y
x
yy
x e e
2 2
0
( ) (1 ) 1(0) .
( )
y y
y
x
y x e y e yy
x e e
ExampleIf find
Sol.
( ) ( )nf x1
( ) ,f xx
11( )f x x
x 2( )f x x
3 3( ) ( 1)( 2) 2f x x x
4 4( ) ( 1)( 2)( 3) 3!f x x x
( ) 1 1( ) ( 1)( 2)( 3) ( ) ( 1) !n n n nf x n x n x
Example Find if
Sol. Using the trigonometry identity
Suppose then
Therefore
( )ny
cos sin( ),2
y x x
sin .y x
cos sin( ),2
cos( ) sin( 2 ).2 2
y x x
( ) sin( ),2
ky x k
( 1) cos( ) sin( ( 1) ).2 2
ky x k x k
( )(sin ) sin( ).2
n nx x
Question Find if
Hint:
Sol.
( )ny 2
1.
2
y
x x
2
1 1 1 1( ).
2 3 1 2
x x x x
( )1 1
( 1) ! 1 1[ ].
3 ( 1) ( 2)
nn
n n
ny
x x