Page 1
Edurev123
8. Laplace and inverse Laplace
Transformation and Properties
8.1 Find the inverse Laplace transform of ?? (?? )=???? (
?? +?? ?? +?? ) .
(2009: 20 Marks)
Solution:
Approach: Use the differentiation property of inverse Laplace transform.
Given:
?? (?? )=ln (
?? +1
?? +?? )
By differentiation property we have
?? -1
[?? (?? )] =
1
?? ?? -1
[?? -1
(?? )]
? ?? -1
[ln (
?? +1
?? +?? )] =
1
?? ?? -1
[
?? ????
ln (
?? +1
?? +?? )]
=
1
?? ?? -1
[
?? ????
ln (?? +1)-ln (?? +?? )]
=
1
?? ?? -1
[
1
?? +1
-
1
?? +?? ]
=
1
?? {?? -1
(
1
?? +1
)-?? -1
(
1
?? +?? )} (Linearity)
=
1
?? {?? -?? -?? -5?? }.
8.2 Use Laplace transform method to solve the following initial value problem:
?? ?? ?? ?? ?? ?? -?? ????
????
+?? =?? ?? ,?? (?? )=?? and
????
????
|
?? =?? =-??
(2011: 15 Marks)
Solution:
The given initial value problem is
Page 2
Edurev123
8. Laplace and inverse Laplace
Transformation and Properties
8.1 Find the inverse Laplace transform of ?? (?? )=???? (
?? +?? ?? +?? ) .
(2009: 20 Marks)
Solution:
Approach: Use the differentiation property of inverse Laplace transform.
Given:
?? (?? )=ln (
?? +1
?? +?? )
By differentiation property we have
?? -1
[?? (?? )] =
1
?? ?? -1
[?? -1
(?? )]
? ?? -1
[ln (
?? +1
?? +?? )] =
1
?? ?? -1
[
?? ????
ln (
?? +1
?? +?? )]
=
1
?? ?? -1
[
?? ????
ln (?? +1)-ln (?? +?? )]
=
1
?? ?? -1
[
1
?? +1
-
1
?? +?? ]
=
1
?? {?? -1
(
1
?? +1
)-?? -1
(
1
?? +?? )} (Linearity)
=
1
?? {?? -?? -?? -5?? }.
8.2 Use Laplace transform method to solve the following initial value problem:
?? ?? ?? ?? ?? ?? -?? ????
????
+?? =?? ?? ,?? (?? )=?? and
????
????
|
?? =?? =-??
(2011: 15 Marks)
Solution:
The given initial value problem is
?? 2
?? ?? ?? 2
-2
????
????
+?? =?? ?? ,?? (0)=2 (??)
(
????
????
)
?? =0
=-1
Taking Laplace transform of both sides of (i), we get
?? (
?? 2
?? ?? ?? 2
-2
????
????
+?? )=?? [?? '
]
? ?? {?? ''
(?? )}-2?? {?? '
(?? )}+?? {?? (?? )}=?? {?? ?? }
? ?? 2
?? {?? (?? )}-???? (0)-?? '
(0)-2[???? {?? (?? )}-?? (0)]+?? {?? (?? )}=
1
??
? ?? (?? )][?? 2
-2?? +1]-?? (2)-(-1)+2(2)=
1
??
? ?? {?? (?? )}(?? -1)
2
=
1
?? +2?? -5
=
1+2?? 2
-5?? ??
? ?? {?? (?? )} =
1+2?? 2
-5?? ?? (?? -1)
2
=
1
?? -1
-
2
(?? -1)
2
+
1
?? … (ii) (By using Partial Fractions)
Taking inverse Laplace of (ii), we get
?? (?? )=?? -1
{
1
?? -1
-
2
(?? -1)
2
+
1
?? }
? ?? (?? )=?? ?? -2?? ?? ?? +1
8.3 Using Laplace transforms, solve the initial value problem
?? ''
+?? ?? +?? =?? -?? ,?? (?? )=-?? ,?? (?? )=??
(2012: 12 Marks)
Solution:
Given :
?? '
+2?? +?? =?? -?? (??)
?? (0) =-1,?? '
(0)=1
Page 3
Edurev123
8. Laplace and inverse Laplace
Transformation and Properties
8.1 Find the inverse Laplace transform of ?? (?? )=???? (
?? +?? ?? +?? ) .
(2009: 20 Marks)
Solution:
Approach: Use the differentiation property of inverse Laplace transform.
Given:
?? (?? )=ln (
?? +1
?? +?? )
By differentiation property we have
?? -1
[?? (?? )] =
1
?? ?? -1
[?? -1
(?? )]
? ?? -1
[ln (
?? +1
?? +?? )] =
1
?? ?? -1
[
?? ????
ln (
?? +1
?? +?? )]
=
1
?? ?? -1
[
?? ????
ln (?? +1)-ln (?? +?? )]
=
1
?? ?? -1
[
1
?? +1
-
1
?? +?? ]
=
1
?? {?? -1
(
1
?? +1
)-?? -1
(
1
?? +?? )} (Linearity)
=
1
?? {?? -?? -?? -5?? }.
8.2 Use Laplace transform method to solve the following initial value problem:
?? ?? ?? ?? ?? ?? -?? ????
????
+?? =?? ?? ,?? (?? )=?? and
????
????
|
?? =?? =-??
(2011: 15 Marks)
Solution:
The given initial value problem is
?? 2
?? ?? ?? 2
-2
????
????
+?? =?? ?? ,?? (0)=2 (??)
(
????
????
)
?? =0
=-1
Taking Laplace transform of both sides of (i), we get
?? (
?? 2
?? ?? ?? 2
-2
????
????
+?? )=?? [?? '
]
? ?? {?? ''
(?? )}-2?? {?? '
(?? )}+?? {?? (?? )}=?? {?? ?? }
? ?? 2
?? {?? (?? )}-???? (0)-?? '
(0)-2[???? {?? (?? )}-?? (0)]+?? {?? (?? )}=
1
??
? ?? (?? )][?? 2
-2?? +1]-?? (2)-(-1)+2(2)=
1
??
? ?? {?? (?? )}(?? -1)
2
=
1
?? +2?? -5
=
1+2?? 2
-5?? ??
? ?? {?? (?? )} =
1+2?? 2
-5?? ?? (?? -1)
2
=
1
?? -1
-
2
(?? -1)
2
+
1
?? … (ii) (By using Partial Fractions)
Taking inverse Laplace of (ii), we get
?? (?? )=?? -1
{
1
?? -1
-
2
(?? -1)
2
+
1
?? }
? ?? (?? )=?? ?? -2?? ?? ?? +1
8.3 Using Laplace transforms, solve the initial value problem
?? ''
+?? ?? +?? =?? -?? ,?? (?? )=-?? ,?? (?? )=??
(2012: 12 Marks)
Solution:
Given :
?? '
+2?? +?? =?? -?? (??)
?? (0) =-1,?? '
(0)=1
Taking Laplace transform of both sides of (i),
?? (?? '
(?? ))+2?? (?? (?? ))+?? (?? )=?? (?? -?? )
??? 2
?? (?? )-???? (0)-?? (0)+2(???? (?? )-?? (0))+?? (?? )=
1
?? +1
??? (?? )(?? 2
+2?? +1)-?? (-1)-1-2(-1)=
1
?? +1
? ?? (?? )(?? +1)
2
+(?? +1)=
1
?? +1
? ?? (?? )=
1
(?? +1)
3
-
1
(?? +1)
(???? )
Taking inverse Laplace transform on both sides of (ii), we have
?? =?? -1
[
1
(?? +1)
3
-
1
(?? +1)
]
? ?? =?? -?? ·
?? 2
2
-?? -??
8.4 By using Laplace transform method solve the differential equation :
(?? ?? +?? ?? )?? =?? ?????? (???? +?? ) , ?? ?? =
?? ?? ?? ?? ?? subject to the initial conditions ?? =?? and
????
????
=??
at ?? =?? in which ?? ,?? and ?? are constants.
(2013 : 15 Marks)
Solution:
(?? 2
+?? 2
)?? =?? sin (???? +?? )
Taking Laplace transform on both sides
?? (
?? 2
?? ?? ?? 2
)+?? 2
?? (?? )=???? [sin ???? cos ?? +cos ???? sin ?? ]
? ?? 2
?? (?? )-???? (0)-?? '
(0)+?? 2
?? (?? )=?? [
?? ?? 2
+?? 2
cos ?? +
?? ?? 2
+?? 2
sin ?? ]
? ?? (?? )=?? [
?? (?? 2
+?? 2
)
2
cos ?? +
?? (?? 2
+?? 2
)
2
sin ?? ]
? ?? =?? {?? -1
[
?? (?? 2
+?? 2
)
2
]cos ?? +?? -1
[
?? (?? 2
+?? 2
)
2
]sin ?? }
Page 4
Edurev123
8. Laplace and inverse Laplace
Transformation and Properties
8.1 Find the inverse Laplace transform of ?? (?? )=???? (
?? +?? ?? +?? ) .
(2009: 20 Marks)
Solution:
Approach: Use the differentiation property of inverse Laplace transform.
Given:
?? (?? )=ln (
?? +1
?? +?? )
By differentiation property we have
?? -1
[?? (?? )] =
1
?? ?? -1
[?? -1
(?? )]
? ?? -1
[ln (
?? +1
?? +?? )] =
1
?? ?? -1
[
?? ????
ln (
?? +1
?? +?? )]
=
1
?? ?? -1
[
?? ????
ln (?? +1)-ln (?? +?? )]
=
1
?? ?? -1
[
1
?? +1
-
1
?? +?? ]
=
1
?? {?? -1
(
1
?? +1
)-?? -1
(
1
?? +?? )} (Linearity)
=
1
?? {?? -?? -?? -5?? }.
8.2 Use Laplace transform method to solve the following initial value problem:
?? ?? ?? ?? ?? ?? -?? ????
????
+?? =?? ?? ,?? (?? )=?? and
????
????
|
?? =?? =-??
(2011: 15 Marks)
Solution:
The given initial value problem is
?? 2
?? ?? ?? 2
-2
????
????
+?? =?? ?? ,?? (0)=2 (??)
(
????
????
)
?? =0
=-1
Taking Laplace transform of both sides of (i), we get
?? (
?? 2
?? ?? ?? 2
-2
????
????
+?? )=?? [?? '
]
? ?? {?? ''
(?? )}-2?? {?? '
(?? )}+?? {?? (?? )}=?? {?? ?? }
? ?? 2
?? {?? (?? )}-???? (0)-?? '
(0)-2[???? {?? (?? )}-?? (0)]+?? {?? (?? )}=
1
??
? ?? (?? )][?? 2
-2?? +1]-?? (2)-(-1)+2(2)=
1
??
? ?? {?? (?? )}(?? -1)
2
=
1
?? +2?? -5
=
1+2?? 2
-5?? ??
? ?? {?? (?? )} =
1+2?? 2
-5?? ?? (?? -1)
2
=
1
?? -1
-
2
(?? -1)
2
+
1
?? … (ii) (By using Partial Fractions)
Taking inverse Laplace of (ii), we get
?? (?? )=?? -1
{
1
?? -1
-
2
(?? -1)
2
+
1
?? }
? ?? (?? )=?? ?? -2?? ?? ?? +1
8.3 Using Laplace transforms, solve the initial value problem
?? ''
+?? ?? +?? =?? -?? ,?? (?? )=-?? ,?? (?? )=??
(2012: 12 Marks)
Solution:
Given :
?? '
+2?? +?? =?? -?? (??)
?? (0) =-1,?? '
(0)=1
Taking Laplace transform of both sides of (i),
?? (?? '
(?? ))+2?? (?? (?? ))+?? (?? )=?? (?? -?? )
??? 2
?? (?? )-???? (0)-?? (0)+2(???? (?? )-?? (0))+?? (?? )=
1
?? +1
??? (?? )(?? 2
+2?? +1)-?? (-1)-1-2(-1)=
1
?? +1
? ?? (?? )(?? +1)
2
+(?? +1)=
1
?? +1
? ?? (?? )=
1
(?? +1)
3
-
1
(?? +1)
(???? )
Taking inverse Laplace transform on both sides of (ii), we have
?? =?? -1
[
1
(?? +1)
3
-
1
(?? +1)
]
? ?? =?? -?? ·
?? 2
2
-?? -??
8.4 By using Laplace transform method solve the differential equation :
(?? ?? +?? ?? )?? =?? ?????? (???? +?? ) , ?? ?? =
?? ?? ?? ?? ?? subject to the initial conditions ?? =?? and
????
????
=??
at ?? =?? in which ?? ,?? and ?? are constants.
(2013 : 15 Marks)
Solution:
(?? 2
+?? 2
)?? =?? sin (???? +?? )
Taking Laplace transform on both sides
?? (
?? 2
?? ?? ?? 2
)+?? 2
?? (?? )=???? [sin ???? cos ?? +cos ???? sin ?? ]
? ?? 2
?? (?? )-???? (0)-?? '
(0)+?? 2
?? (?? )=?? [
?? ?? 2
+?? 2
cos ?? +
?? ?? 2
+?? 2
sin ?? ]
? ?? (?? )=?? [
?? (?? 2
+?? 2
)
2
cos ?? +
?? (?? 2
+?? 2
)
2
sin ?? ]
? ?? =?? {?? -1
[
?? (?? 2
+?? 2
)
2
]cos ?? +?? -1
[
?? (?? 2
+?? 2
)
2
]sin ?? }
Now
?? ????
1
(?? 2
+?? 2
)
=-
2?? (?? 2
+?? 2
)
2
? ?? -1
(
2?? (?? 2
+?? 2
)
2
)=(-1)?? ?? -1
(
1
?? 2
+?? 2
)
=
-?? ?? ·sin ????
? ?? -1
(
?? (?? 2
+?? 2
)
2
)=
?? 2?? sin ????
Let
?? (?? ) =
1
?? 2
+?? 2
;?? (?? )=
1
?? 2
+?? 2
?? (?? ) =?? -1
(?? (?? ))=
1
?? sin ????
?? (?? ) =?? -1
(?? (?? ))=
1
?? sin ????
?? -1
(?? (?? )?? (?? )) =?? *?? =??
?? 0
??? (?? )?? (?? -?? )????
=
1
?? 2
??
?? 0
?sin ???? sin ?? (?? -?? )????
=
1
2?? 2
??
?? 0
?(cos ?? (?? -2?? )-cos ???? )????
=
1
2?? 2
[
sin ?? (?? -2?? )
-2?? -?? cos ???? ]
0
?? =
1
2?? 2
[
-sin ????
-2?? +
sin ????
2?? -?? cos ???? ]=
1
2?? 2
[
sin ????
?? -?? cos ???? ]
?? =?? {
1
2?? [
sin ????
?? -?? cos ???? ]cos ?? +(
?? 2?? sin ???? sin ?? )}
=?? {
1
2?? 2
sin ???? cos ?? -
?? 2?? cos (???? +?? )}
=
?? 2?? 2
{sin ???? cos ?? -???? cos (???? +?? )}
8.5 Solve the initial value problem
?? ?? ?? ?? ?? ?? +?? =?? ?? -?? ?? ?????? ?? ,?? (?? )=?? ,?? (?? )=??
by using Laplace-transform.
(2014 : 20 Marks)
Solution:
Given equation is
Page 5
Edurev123
8. Laplace and inverse Laplace
Transformation and Properties
8.1 Find the inverse Laplace transform of ?? (?? )=???? (
?? +?? ?? +?? ) .
(2009: 20 Marks)
Solution:
Approach: Use the differentiation property of inverse Laplace transform.
Given:
?? (?? )=ln (
?? +1
?? +?? )
By differentiation property we have
?? -1
[?? (?? )] =
1
?? ?? -1
[?? -1
(?? )]
? ?? -1
[ln (
?? +1
?? +?? )] =
1
?? ?? -1
[
?? ????
ln (
?? +1
?? +?? )]
=
1
?? ?? -1
[
?? ????
ln (?? +1)-ln (?? +?? )]
=
1
?? ?? -1
[
1
?? +1
-
1
?? +?? ]
=
1
?? {?? -1
(
1
?? +1
)-?? -1
(
1
?? +?? )} (Linearity)
=
1
?? {?? -?? -?? -5?? }.
8.2 Use Laplace transform method to solve the following initial value problem:
?? ?? ?? ?? ?? ?? -?? ????
????
+?? =?? ?? ,?? (?? )=?? and
????
????
|
?? =?? =-??
(2011: 15 Marks)
Solution:
The given initial value problem is
?? 2
?? ?? ?? 2
-2
????
????
+?? =?? ?? ,?? (0)=2 (??)
(
????
????
)
?? =0
=-1
Taking Laplace transform of both sides of (i), we get
?? (
?? 2
?? ?? ?? 2
-2
????
????
+?? )=?? [?? '
]
? ?? {?? ''
(?? )}-2?? {?? '
(?? )}+?? {?? (?? )}=?? {?? ?? }
? ?? 2
?? {?? (?? )}-???? (0)-?? '
(0)-2[???? {?? (?? )}-?? (0)]+?? {?? (?? )}=
1
??
? ?? (?? )][?? 2
-2?? +1]-?? (2)-(-1)+2(2)=
1
??
? ?? {?? (?? )}(?? -1)
2
=
1
?? +2?? -5
=
1+2?? 2
-5?? ??
? ?? {?? (?? )} =
1+2?? 2
-5?? ?? (?? -1)
2
=
1
?? -1
-
2
(?? -1)
2
+
1
?? … (ii) (By using Partial Fractions)
Taking inverse Laplace of (ii), we get
?? (?? )=?? -1
{
1
?? -1
-
2
(?? -1)
2
+
1
?? }
? ?? (?? )=?? ?? -2?? ?? ?? +1
8.3 Using Laplace transforms, solve the initial value problem
?? ''
+?? ?? +?? =?? -?? ,?? (?? )=-?? ,?? (?? )=??
(2012: 12 Marks)
Solution:
Given :
?? '
+2?? +?? =?? -?? (??)
?? (0) =-1,?? '
(0)=1
Taking Laplace transform of both sides of (i),
?? (?? '
(?? ))+2?? (?? (?? ))+?? (?? )=?? (?? -?? )
??? 2
?? (?? )-???? (0)-?? (0)+2(???? (?? )-?? (0))+?? (?? )=
1
?? +1
??? (?? )(?? 2
+2?? +1)-?? (-1)-1-2(-1)=
1
?? +1
? ?? (?? )(?? +1)
2
+(?? +1)=
1
?? +1
? ?? (?? )=
1
(?? +1)
3
-
1
(?? +1)
(???? )
Taking inverse Laplace transform on both sides of (ii), we have
?? =?? -1
[
1
(?? +1)
3
-
1
(?? +1)
]
? ?? =?? -?? ·
?? 2
2
-?? -??
8.4 By using Laplace transform method solve the differential equation :
(?? ?? +?? ?? )?? =?? ?????? (???? +?? ) , ?? ?? =
?? ?? ?? ?? ?? subject to the initial conditions ?? =?? and
????
????
=??
at ?? =?? in which ?? ,?? and ?? are constants.
(2013 : 15 Marks)
Solution:
(?? 2
+?? 2
)?? =?? sin (???? +?? )
Taking Laplace transform on both sides
?? (
?? 2
?? ?? ?? 2
)+?? 2
?? (?? )=???? [sin ???? cos ?? +cos ???? sin ?? ]
? ?? 2
?? (?? )-???? (0)-?? '
(0)+?? 2
?? (?? )=?? [
?? ?? 2
+?? 2
cos ?? +
?? ?? 2
+?? 2
sin ?? ]
? ?? (?? )=?? [
?? (?? 2
+?? 2
)
2
cos ?? +
?? (?? 2
+?? 2
)
2
sin ?? ]
? ?? =?? {?? -1
[
?? (?? 2
+?? 2
)
2
]cos ?? +?? -1
[
?? (?? 2
+?? 2
)
2
]sin ?? }
Now
?? ????
1
(?? 2
+?? 2
)
=-
2?? (?? 2
+?? 2
)
2
? ?? -1
(
2?? (?? 2
+?? 2
)
2
)=(-1)?? ?? -1
(
1
?? 2
+?? 2
)
=
-?? ?? ·sin ????
? ?? -1
(
?? (?? 2
+?? 2
)
2
)=
?? 2?? sin ????
Let
?? (?? ) =
1
?? 2
+?? 2
;?? (?? )=
1
?? 2
+?? 2
?? (?? ) =?? -1
(?? (?? ))=
1
?? sin ????
?? (?? ) =?? -1
(?? (?? ))=
1
?? sin ????
?? -1
(?? (?? )?? (?? )) =?? *?? =??
?? 0
??? (?? )?? (?? -?? )????
=
1
?? 2
??
?? 0
?sin ???? sin ?? (?? -?? )????
=
1
2?? 2
??
?? 0
?(cos ?? (?? -2?? )-cos ???? )????
=
1
2?? 2
[
sin ?? (?? -2?? )
-2?? -?? cos ???? ]
0
?? =
1
2?? 2
[
-sin ????
-2?? +
sin ????
2?? -?? cos ???? ]=
1
2?? 2
[
sin ????
?? -?? cos ???? ]
?? =?? {
1
2?? [
sin ????
?? -?? cos ???? ]cos ?? +(
?? 2?? sin ???? sin ?? )}
=?? {
1
2?? 2
sin ???? cos ?? -
?? 2?? cos (???? +?? )}
=
?? 2?? 2
{sin ???? cos ?? -???? cos (???? +?? )}
8.5 Solve the initial value problem
?? ?? ?? ?? ?? ?? +?? =?? ?? -?? ?? ?????? ?? ,?? (?? )=?? ,?? (?? )=??
by using Laplace-transform.
(2014 : 20 Marks)
Solution:
Given equation is
?? 2
?? ?? ?? 2
+?? =8?? -2?? sin ??
? ?? '
+?? =8?? -2?? sin?? (??)
? Taking Laplace transform of both sides of (??) , we get
?? (?? '
)+?? (?? )=8?? (?? -2?? sin ?? )
? ?? 2
?? {(?? )}-???? (0)-?? '
(0)+?? (?? (?? )=
8
(?? +2)
2
+1
? ?? 2
?? {(?? )}+?? {?? (?? )}=
8
?? 2
+4?? +5
? ?? (?? (?? ))(?? 2
+1)=
8
?? 2
+4?? +5
? ?? {?? (?? )}=
8
(?? 2
+1)(?? 2
+4?? +5)
? ?? (?? )=?? -1
8
(?? 2
+1)(?? 2
+4?? +5)
?? (?? )=?? -1
[
-?? +1
?? 2
+1
+
?? +3
?? 2
+4?? +5
]
=?? -1
(
-?? ?? 2
+1
)+?? -1
(
1
?? 2
+1
)+?? -1
(
(?? +2)+1
(?? +2)
2
+1
)
=-cos ?? +sin ?? +?? -2?? ?? -1
(
?? +1
?? 2
+1
)
=-cos ?? +sin ?? +?? -2?? {?? -1
(
?? ?? 2
+1
)··-1(
1
?? 2
+1
)}
=-cos ?? +sin ?? +?? -2?? cos ?? +?? -2?? sin ?? =(?? -2?? -1)cos ?? +(?? -2?? +1)sin ??
which is the required solution.
8.6
(i) Obtain Laplace Inverse transform of
?? (?? )={???? (?? +
?? ?? ?? )+
?? ?? ?? +????
?? -????
}
(2015 : 6 Marks)
(ii) Using Laplace transform, solve:
?? ''
+?? =?? ,?? (?? )=?? ,?? (?? )=-??
(2015 : 6 Marks)
Solution:
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