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| Boilerpipe Text | Higher Order Derivatives
Since
f
x
and
f
y
are functions of
x
and
y
,
we can take derivatives of
f
x
and
f
y
to
get second derivatives
.Ā There are four such
second derivatives, since each time we can differentiate with
respect to
x
or
y
.Ā Each of these second derivatives has
multiple notations, and we have listed some of them.
Notation for second partial
derivatives
(
f
x
)
x
=
f
x
x
=
f
11
=
ā
ā
x
(
ā
f
ā
x
)
=
ā
2
f
ā
x
2
=
ā
2
z
ā
x
2
(
f
x
)
y
=
f
x
y
=
f
12
=
ā
ā
y
(
ā
f
ā
x
)
=
ā
2
f
ā
y
ā
x
=
ā
2
z
ā
y
ā
x
(
f
y
)
x
=
f
y
x
=
f
21
=
ā
ā
x
(
ā
f
ā
y
)
=
ā
2
f
ā
x
ā
y
=
ā
2
z
ā
x
ā
y
(
f
y
)
y
=
f
y
y
=
f
22
=
ā
ā
y
(
ā
f
ā
y
)
=
ā
2
f
ā
y
2
=
ā
2
z
ā
y
2
A nice result regarding second partial derivatives is
Clairaut's Theorem
, which tells us
that the mixed variable partial derivatives are equal.
Clairaut's Theorem
If
f
x
y
and
f
y
x
are both defined and continuous in
a region containing the point
(
a
,
b
)
, then
f
x
y
(
a
,
b
)
=
f
y
x
(
a
,
b
)
.
A consequence of this theorem is that we don't need to keep
track of the order in which we take derivatives.Ā
Example 1
:Ā Let
f
(
x
,
y
)
=
3
x
2
ā
4
y
3
ā
7
x
2
y
3
.Ā
Previously, we determined that
f
x
=
6
x
ā
14
x
y
3
and
f
y
=
ā
12
y
2
ā
21
x
2
y
2
.Ā We have four second derivatives, but
as Clairaut's Theorem tells us,
f
x
y
=
f
y
x
, so we really only
need to compute three of them (we do all four to illustrate the
theorem).
Solution 1
:
(
f
x
)
x
=
ā
ā
x
f
x
=
ā
ā
x
(
6
x
ā
14
x
y
3
)
=
6
ā
14
y
3
(
f
x
)
y
=
ā
ā
y
f
x
=
ā
ā
y
(
6
x
ā
14
x
y
3
)
=
0
ā
42
x
y
2
=
ā
42
x
y
2
(
f
y
)
x
=
ā
ā
x
f
y
=
ā
ā
x
(
12
y
2
ā
21
x
2
y
2
)
=
0
ā
42
x
y
2
=
ā
42
x
y
2
(
f
y
)
y
=
ā
ā
y
f
y
=
ā
ā
y
(
12
y
2
ā
21
x
2
y
2
)
=
24
y
ā
42
x
2
y
Higher partial derivatives and Clairaut's theorem are explained
in the following video. |
| Markdown | | | | |
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| | | |
| **Notation for second partial derivatives** (fx)x\=fxx\=f11\=āāx(āfāx)\=ā2fāx2\=ā2zāx2 ( f x ) x \= f x x \= f 11 \= ā ā x ( ā f ā x ) \= ā 2 f ā x 2 \= ā 2 z ā x 2 (fx)y\=fxy\=f12\=āāy(āfāx)\=ā2fāyāx\=ā2zāyāx ( f x ) y \= f x y \= f 12 \= ā ā y ( ā f ā x ) \= ā 2 f ā y ā x \= ā 2 z ā y ā x (fy)x\=fyx\=f21\=āāx(āfāy)\=ā2fāxāy\=ā2zāxāy ( f y ) x \= f y x \= f 21 \= ā ā x ( ā f ā y ) \= ā 2 f ā x ā y \= ā 2 z ā x ā y (fy)y\=fyy\=f22\=āāy(āfāy)\=ā2fāy2\=ā2zāy2 ( f y ) y \= f y y \= f 22 \= ā ā y ( ā f ā y ) \= ā 2 f ā y 2 \= ā 2 z ā y 2 | | |
| | | |
| **Clairaut's Theorem** If fxy f x y and fyx f y x are both defined and continuous in a region containing the point (a,b) ( a , b ), then fxy(a,b)\=fyx(a,b). f x y ( a , b ) \= f y x ( a , b ) . | | | |
| Readable Markdown | ### Higher Order Derivatives
Since f x and f y are functions of x and y, **we can take derivatives of f x and f y to get second derivatives**. There are four such second derivatives, since each time we can differentiate with respect to x or y. Each of these second derivatives has multiple notations, and we have listed some of them.
**Notation for second partial derivatives**
( f x ) x \= f x x \= f 11 \= ā ā x ( ā f ā x ) \= ā 2 f ā x 2 \= ā 2 z ā x 2
( f x ) y \= f x y \= f 12 \= ā ā y ( ā f ā x ) \= ā 2 f ā y ā x \= ā 2 z ā y ā x
( f y ) x \= f y x \= f 21 \= ā ā x ( ā f ā y ) \= ā 2 f ā x ā y \= ā 2 z ā x ā y
( f y ) y \= f y y \= f 22 \= ā ā y ( ā f ā y ) \= ā 2 f ā y 2 \= ā 2 z ā y 2
A nice result regarding second partial derivatives is **Clairaut's Theorem**, which tells us that the mixed variable partial derivatives are equal.
**Clairaut's Theorem**
If
f
x
y
and
f
y
x
are both defined and continuous in a region containing the point
(
a
,
b
)
, then
f x y ( a , b ) \= f y x ( a , b ) .
A consequence of this theorem is that we don't need to keep track of the order in which we take derivatives.
**Example 1**: Let f ( x , y ) \= 3 x 2 ā 4 y 3 ā 7 x 2 y 3. Previously, we determined that f x \= 6 x ā 14 x y 3 and f y \= ā 12 y 2 ā 21 x 2 y 2. We have four second derivatives, but as Clairaut's Theorem tells us, f x y \= f y x, so we really only need to compute three of them (we do all four to illustrate the theorem).
**Solution 1**:
( f x ) x \= ā ā x f x \= ā ā x ( 6 x ā 14 x y 3 ) \= 6 ā 14 y 3 ( f x ) y \= ā ā y f x \= ā ā y ( 6 x ā 14 x y 3 ) \= 0 ā 42 x y 2 \= ā 42 x y 2 ( f y ) x \= ā ā x f y \= ā ā x ( 12 y 2 ā 21 x 2 y 2 ) \= 0 ā 42 x y 2 \= ā 42 x y 2 ( f y ) y \= ā ā y f y \= ā ā y ( 12 y 2 ā 21 x 2 y 2 ) \= 24 y ā 42 x 2 y
Higher partial derivatives and Clairaut's theorem are explained in the following video. |
| Shard | 65 (laksa) |
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| Unparsed URL | edu,utexas!ma,web,/users/m408s/m408d/CurrentWeb/LM14-3-8.php s443 |