Page 1
Edurev123
7. Determination of Complete Solution
when one solution is known and Method of
Variation of Parameter
7.1 Using the method of variation of parameters, solve the second order
differential equation
?? ?? ?? ?? ?? ?? +?? ?? =?????? ?? ??
(2011: 15 Marks)
Solution:
The given equation is
?? 2
?? ?? ?? 2
+4?? =tan 2?? ? (?? 2
+4)?? =tan 2?? ,?? =
?? ????
?????????????????? ???????????????? ???? ?? 2
+?? =0
? ?? =±2?? ? Complementary function is
?? ?? =?? 1
cos 2?? +?? 2
sin 2?? , where ?? 1
and ?? 2
are arbitrary constants.
Let ?? ?? =?? cos 2?? +?? sin 2?? be particular integral of (i) where ?? and ?? are functions of ?? .
Then, ?? (?? )=cos 2?? ,?? (?? )=sin 2?? and ?? (?? )=tan 2?? ?? =?
4?? ?? (?? ,?? )
????
=
1
2
? cos 2?? ·tan 2?????? =
1
2
? sin 2?????? =-
1
4
cos 2?? ?? ?? =-
1
4
[log |sec 2?? +tan 2?? |-sin 2?? ]cos 2?? -
1
4
cos 2?? ·sin 2?? =-
1
4
(log |sec 2?? +tan 2?? |)cos 2??
? The required general solution is
?? =?? ?? +?? ?? =?? 1
cos 2?? +?? 2
sin 2?? -
1
4
(log |sec 2?? +tan 2?? |)cos 2??
7.2 Using the method of variation of parameter solve the differential equation :
Page 2
Edurev123
7. Determination of Complete Solution
when one solution is known and Method of
Variation of Parameter
7.1 Using the method of variation of parameters, solve the second order
differential equation
?? ?? ?? ?? ?? ?? +?? ?? =?????? ?? ??
(2011: 15 Marks)
Solution:
The given equation is
?? 2
?? ?? ?? 2
+4?? =tan 2?? ? (?? 2
+4)?? =tan 2?? ,?? =
?? ????
?????????????????? ???????????????? ???? ?? 2
+?? =0
? ?? =±2?? ? Complementary function is
?? ?? =?? 1
cos 2?? +?? 2
sin 2?? , where ?? 1
and ?? 2
are arbitrary constants.
Let ?? ?? =?? cos 2?? +?? sin 2?? be particular integral of (i) where ?? and ?? are functions of ?? .
Then, ?? (?? )=cos 2?? ,?? (?? )=sin 2?? and ?? (?? )=tan 2?? ?? =?
4?? ?? (?? ,?? )
????
=
1
2
? cos 2?? ·tan 2?????? =
1
2
? sin 2?????? =-
1
4
cos 2?? ?? ?? =-
1
4
[log |sec 2?? +tan 2?? |-sin 2?? ]cos 2?? -
1
4
cos 2?? ·sin 2?? =-
1
4
(log |sec 2?? +tan 2?? |)cos 2??
? The required general solution is
?? =?? ?? +?? ?? =?? 1
cos 2?? +?? 2
sin 2?? -
1
4
(log |sec 2?? +tan 2?? |)cos 2??
7.2 Using the method of variation of parameter solve the differential equation :
?? ?? ?? ?? ?? ?? +?? ?? ?? =?????? ????
(2013 : 10 Marks)
Solution:
Homogeneous part is
?? 2
?? ?? ?? 2
+?? 2
?? =0?(?? 2
+?? 2
)?? =0
Auxiliary equation: ?? 2
+?? 2
=0??? =±????
? ?? =cos ????
?? =sin ????
are two independent solutions of homogeneous equation.
Let ?? =???? +???? complete the solution.
Then,
?? =?
-????
?? ?? =?
?? ?? ??
where
?? = Wronskian (?? ,?? )
=|
cos ???? sin ????
-?? sin ???? ?? cos ????
|=??
? ?? =-?
sin ???? ·sec ????
?? ???? =-
1
?? ?tan ????????
=
1
?? 2
?
-?? sin ????
cos ?? ?? ???? =
1
?? 2
ln (cos ???? )+?? 1
?? =?
cos ???? sec ????
?? ???? =
?? ?? +?? 2
? ?? =?? 1
cos ???? +
1
?? 2
cos ???? ×ln (cos (???? ))+?? 2
sin ???? +
?? ?? sin ????
7.3 Solve by the method of variation of parameters :
????
????
-?? ?? =?????? ??
(2014 : 10 Marks)
Page 3
Edurev123
7. Determination of Complete Solution
when one solution is known and Method of
Variation of Parameter
7.1 Using the method of variation of parameters, solve the second order
differential equation
?? ?? ?? ?? ?? ?? +?? ?? =?????? ?? ??
(2011: 15 Marks)
Solution:
The given equation is
?? 2
?? ?? ?? 2
+4?? =tan 2?? ? (?? 2
+4)?? =tan 2?? ,?? =
?? ????
?????????????????? ???????????????? ???? ?? 2
+?? =0
? ?? =±2?? ? Complementary function is
?? ?? =?? 1
cos 2?? +?? 2
sin 2?? , where ?? 1
and ?? 2
are arbitrary constants.
Let ?? ?? =?? cos 2?? +?? sin 2?? be particular integral of (i) where ?? and ?? are functions of ?? .
Then, ?? (?? )=cos 2?? ,?? (?? )=sin 2?? and ?? (?? )=tan 2?? ?? =?
4?? ?? (?? ,?? )
????
=
1
2
? cos 2?? ·tan 2?????? =
1
2
? sin 2?????? =-
1
4
cos 2?? ?? ?? =-
1
4
[log |sec 2?? +tan 2?? |-sin 2?? ]cos 2?? -
1
4
cos 2?? ·sin 2?? =-
1
4
(log |sec 2?? +tan 2?? |)cos 2??
? The required general solution is
?? =?? ?? +?? ?? =?? 1
cos 2?? +?? 2
sin 2?? -
1
4
(log |sec 2?? +tan 2?? |)cos 2??
7.2 Using the method of variation of parameter solve the differential equation :
?? ?? ?? ?? ?? ?? +?? ?? ?? =?????? ????
(2013 : 10 Marks)
Solution:
Homogeneous part is
?? 2
?? ?? ?? 2
+?? 2
?? =0?(?? 2
+?? 2
)?? =0
Auxiliary equation: ?? 2
+?? 2
=0??? =±????
? ?? =cos ????
?? =sin ????
are two independent solutions of homogeneous equation.
Let ?? =???? +???? complete the solution.
Then,
?? =?
-????
?? ?? =?
?? ?? ??
where
?? = Wronskian (?? ,?? )
=|
cos ???? sin ????
-?? sin ???? ?? cos ????
|=??
? ?? =-?
sin ???? ·sec ????
?? ???? =-
1
?? ?tan ????????
=
1
?? 2
?
-?? sin ????
cos ?? ?? ???? =
1
?? 2
ln (cos ???? )+?? 1
?? =?
cos ???? sec ????
?? ???? =
?? ?? +?? 2
? ?? =?? 1
cos ???? +
1
?? 2
cos ???? ×ln (cos (???? ))+?? 2
sin ???? +
?? ?? sin ????
7.3 Solve by the method of variation of parameters :
????
????
-?? ?? =?????? ??
(2014 : 10 Marks)
Solution:
Given that,
????
????
-5?? =sin ?? (??)
Differentiating (i) w.r.t. ?? , we get
?? 2
?? ?? ?? 2
-5
????
????
=cos ??
(?? 2
-5?? )?? =cos ??
Now consider auxiliary equation of (ii)
?? (?? -5)=0
? ?? =0,5
? C.F. of (ii) is. ?? ?? =?? 1
+?? 2
?? 5??
Let ?? ?? =???? +???? be a particular integral of (ii) where ?? and ?? are functions of ?? and ?? =
1,?? =?? 5??
Now ?? =|
?? ?? ?? '
?? '
|-|
1 ?? 5?? 0 5?? 5?? |=5?? 5?? ?0
? ?? =-?
?? ?? ?? =-?
?? 5?? ·cos ?????? 5?? 5?? =-
1
5
? cos ?????? =-
1
5
sin:??
?? =?
????
?? =?
1·cos ?? 5?? 5?? ???? =
1
5
? ?? -5?? cos ?????? =
1
5
?? -5?? 25+1
[-5cos ?? +sin ?? ]
=
1?? -5?? 5
-5cos ?? +sin ?? ]
26
[-5
? The general solution of (ii) is
?? =?? ?? +?? ?? ?? =?? 1
+?? 2
?? 5?? -
1
5
sin ?? ·1+
1
5
?? -5?? 26
[-5cos ?? +sin ?? ]
?? =?? 1
+?? 2
?? 5?? -
1
26
[cos ?? +5sin ?? ]
which is the required solution of given equation.
7.4 Solve the following differential equations:
Page 4
Edurev123
7. Determination of Complete Solution
when one solution is known and Method of
Variation of Parameter
7.1 Using the method of variation of parameters, solve the second order
differential equation
?? ?? ?? ?? ?? ?? +?? ?? =?????? ?? ??
(2011: 15 Marks)
Solution:
The given equation is
?? 2
?? ?? ?? 2
+4?? =tan 2?? ? (?? 2
+4)?? =tan 2?? ,?? =
?? ????
?????????????????? ???????????????? ???? ?? 2
+?? =0
? ?? =±2?? ? Complementary function is
?? ?? =?? 1
cos 2?? +?? 2
sin 2?? , where ?? 1
and ?? 2
are arbitrary constants.
Let ?? ?? =?? cos 2?? +?? sin 2?? be particular integral of (i) where ?? and ?? are functions of ?? .
Then, ?? (?? )=cos 2?? ,?? (?? )=sin 2?? and ?? (?? )=tan 2?? ?? =?
4?? ?? (?? ,?? )
????
=
1
2
? cos 2?? ·tan 2?????? =
1
2
? sin 2?????? =-
1
4
cos 2?? ?? ?? =-
1
4
[log |sec 2?? +tan 2?? |-sin 2?? ]cos 2?? -
1
4
cos 2?? ·sin 2?? =-
1
4
(log |sec 2?? +tan 2?? |)cos 2??
? The required general solution is
?? =?? ?? +?? ?? =?? 1
cos 2?? +?? 2
sin 2?? -
1
4
(log |sec 2?? +tan 2?? |)cos 2??
7.2 Using the method of variation of parameter solve the differential equation :
?? ?? ?? ?? ?? ?? +?? ?? ?? =?????? ????
(2013 : 10 Marks)
Solution:
Homogeneous part is
?? 2
?? ?? ?? 2
+?? 2
?? =0?(?? 2
+?? 2
)?? =0
Auxiliary equation: ?? 2
+?? 2
=0??? =±????
? ?? =cos ????
?? =sin ????
are two independent solutions of homogeneous equation.
Let ?? =???? +???? complete the solution.
Then,
?? =?
-????
?? ?? =?
?? ?? ??
where
?? = Wronskian (?? ,?? )
=|
cos ???? sin ????
-?? sin ???? ?? cos ????
|=??
? ?? =-?
sin ???? ·sec ????
?? ???? =-
1
?? ?tan ????????
=
1
?? 2
?
-?? sin ????
cos ?? ?? ???? =
1
?? 2
ln (cos ???? )+?? 1
?? =?
cos ???? sec ????
?? ???? =
?? ?? +?? 2
? ?? =?? 1
cos ???? +
1
?? 2
cos ???? ×ln (cos (???? ))+?? 2
sin ???? +
?? ?? sin ????
7.3 Solve by the method of variation of parameters :
????
????
-?? ?? =?????? ??
(2014 : 10 Marks)
Solution:
Given that,
????
????
-5?? =sin ?? (??)
Differentiating (i) w.r.t. ?? , we get
?? 2
?? ?? ?? 2
-5
????
????
=cos ??
(?? 2
-5?? )?? =cos ??
Now consider auxiliary equation of (ii)
?? (?? -5)=0
? ?? =0,5
? C.F. of (ii) is. ?? ?? =?? 1
+?? 2
?? 5??
Let ?? ?? =???? +???? be a particular integral of (ii) where ?? and ?? are functions of ?? and ?? =
1,?? =?? 5??
Now ?? =|
?? ?? ?? '
?? '
|-|
1 ?? 5?? 0 5?? 5?? |=5?? 5?? ?0
? ?? =-?
?? ?? ?? =-?
?? 5?? ·cos ?????? 5?? 5?? =-
1
5
? cos ?????? =-
1
5
sin:??
?? =?
????
?? =?
1·cos ?? 5?? 5?? ???? =
1
5
? ?? -5?? cos ?????? =
1
5
?? -5?? 25+1
[-5cos ?? +sin ?? ]
=
1?? -5?? 5
-5cos ?? +sin ?? ]
26
[-5
? The general solution of (ii) is
?? =?? ?? +?? ?? ?? =?? 1
+?? 2
?? 5?? -
1
5
sin ?? ·1+
1
5
?? -5?? 26
[-5cos ?? +sin ?? ]
?? =?? 1
+?? 2
?? 5?? -
1
26
[cos ?? +5sin ?? ]
which is the required solution of given equation.
7.4 Solve the following differential equations:
?? ?? ?? ?? ?? ?? ?? -?? (?? +?? )
????
????
+(?? +?? )?? =(?? -?? )?? ?? ??
when ?? ?? is a solution to its corresponding homogeneous differential equation.
(2014 : 15 Marks)
Solution:
Given equation is
?? ?? ''
-2(?? +1)?? +(?? +2)?? =(?? -2)?? 2?? (??)
It is given that ?? ?? is a solution to its corresponding homogeneous differential equation,
i.e., ?? =?? =?? ?? is the part of C.F. of (i).
Let the general solution of (i) is ?? =???? . Then ?? is given by
?? 2
?? ?? ?? 2
+(?? +
2????
?? ????
)
????
????
=
?? ??
where
?? =
-2(1+?? )
?? ,?? =
?? +2
?? ,?? =
(?? -2)
?? ?? 2??
?
?? 2
?? ?? ?? 2
+(-
2
?? (?? +?? )+
2
?? ?? (?? ?? ))
????
????
=
(?? -2)
?? ?? 2?? ?? ??
?
?? 2
?? ?? ?? 2
+(-
2
?? -2+2)
????
????
=
(?? -2)
?? ?? ?? (???? )
Let
????
????
=?? ?
?? 2
?? ?? ?? 2
=
????
????
? from (ii), we have
????
????
+(-
2
?? )?? =
(?? -2)
?? ?? ??
which lies linear in ?? .
Page 5
Edurev123
7. Determination of Complete Solution
when one solution is known and Method of
Variation of Parameter
7.1 Using the method of variation of parameters, solve the second order
differential equation
?? ?? ?? ?? ?? ?? +?? ?? =?????? ?? ??
(2011: 15 Marks)
Solution:
The given equation is
?? 2
?? ?? ?? 2
+4?? =tan 2?? ? (?? 2
+4)?? =tan 2?? ,?? =
?? ????
?????????????????? ???????????????? ???? ?? 2
+?? =0
? ?? =±2?? ? Complementary function is
?? ?? =?? 1
cos 2?? +?? 2
sin 2?? , where ?? 1
and ?? 2
are arbitrary constants.
Let ?? ?? =?? cos 2?? +?? sin 2?? be particular integral of (i) where ?? and ?? are functions of ?? .
Then, ?? (?? )=cos 2?? ,?? (?? )=sin 2?? and ?? (?? )=tan 2?? ?? =?
4?? ?? (?? ,?? )
????
=
1
2
? cos 2?? ·tan 2?????? =
1
2
? sin 2?????? =-
1
4
cos 2?? ?? ?? =-
1
4
[log |sec 2?? +tan 2?? |-sin 2?? ]cos 2?? -
1
4
cos 2?? ·sin 2?? =-
1
4
(log |sec 2?? +tan 2?? |)cos 2??
? The required general solution is
?? =?? ?? +?? ?? =?? 1
cos 2?? +?? 2
sin 2?? -
1
4
(log |sec 2?? +tan 2?? |)cos 2??
7.2 Using the method of variation of parameter solve the differential equation :
?? ?? ?? ?? ?? ?? +?? ?? ?? =?????? ????
(2013 : 10 Marks)
Solution:
Homogeneous part is
?? 2
?? ?? ?? 2
+?? 2
?? =0?(?? 2
+?? 2
)?? =0
Auxiliary equation: ?? 2
+?? 2
=0??? =±????
? ?? =cos ????
?? =sin ????
are two independent solutions of homogeneous equation.
Let ?? =???? +???? complete the solution.
Then,
?? =?
-????
?? ?? =?
?? ?? ??
where
?? = Wronskian (?? ,?? )
=|
cos ???? sin ????
-?? sin ???? ?? cos ????
|=??
? ?? =-?
sin ???? ·sec ????
?? ???? =-
1
?? ?tan ????????
=
1
?? 2
?
-?? sin ????
cos ?? ?? ???? =
1
?? 2
ln (cos ???? )+?? 1
?? =?
cos ???? sec ????
?? ???? =
?? ?? +?? 2
? ?? =?? 1
cos ???? +
1
?? 2
cos ???? ×ln (cos (???? ))+?? 2
sin ???? +
?? ?? sin ????
7.3 Solve by the method of variation of parameters :
????
????
-?? ?? =?????? ??
(2014 : 10 Marks)
Solution:
Given that,
????
????
-5?? =sin ?? (??)
Differentiating (i) w.r.t. ?? , we get
?? 2
?? ?? ?? 2
-5
????
????
=cos ??
(?? 2
-5?? )?? =cos ??
Now consider auxiliary equation of (ii)
?? (?? -5)=0
? ?? =0,5
? C.F. of (ii) is. ?? ?? =?? 1
+?? 2
?? 5??
Let ?? ?? =???? +???? be a particular integral of (ii) where ?? and ?? are functions of ?? and ?? =
1,?? =?? 5??
Now ?? =|
?? ?? ?? '
?? '
|-|
1 ?? 5?? 0 5?? 5?? |=5?? 5?? ?0
? ?? =-?
?? ?? ?? =-?
?? 5?? ·cos ?????? 5?? 5?? =-
1
5
? cos ?????? =-
1
5
sin:??
?? =?
????
?? =?
1·cos ?? 5?? 5?? ???? =
1
5
? ?? -5?? cos ?????? =
1
5
?? -5?? 25+1
[-5cos ?? +sin ?? ]
=
1?? -5?? 5
-5cos ?? +sin ?? ]
26
[-5
? The general solution of (ii) is
?? =?? ?? +?? ?? ?? =?? 1
+?? 2
?? 5?? -
1
5
sin ?? ·1+
1
5
?? -5?? 26
[-5cos ?? +sin ?? ]
?? =?? 1
+?? 2
?? 5?? -
1
26
[cos ?? +5sin ?? ]
which is the required solution of given equation.
7.4 Solve the following differential equations:
?? ?? ?? ?? ?? ?? ?? -?? (?? +?? )
????
????
+(?? +?? )?? =(?? -?? )?? ?? ??
when ?? ?? is a solution to its corresponding homogeneous differential equation.
(2014 : 15 Marks)
Solution:
Given equation is
?? ?? ''
-2(?? +1)?? +(?? +2)?? =(?? -2)?? 2?? (??)
It is given that ?? ?? is a solution to its corresponding homogeneous differential equation,
i.e., ?? =?? =?? ?? is the part of C.F. of (i).
Let the general solution of (i) is ?? =???? . Then ?? is given by
?? 2
?? ?? ?? 2
+(?? +
2????
?? ????
)
????
????
=
?? ??
where
?? =
-2(1+?? )
?? ,?? =
?? +2
?? ,?? =
(?? -2)
?? ?? 2??
?
?? 2
?? ?? ?? 2
+(-
2
?? (?? +?? )+
2
?? ?? (?? ?? ))
????
????
=
(?? -2)
?? ?? 2?? ?? ??
?
?? 2
?? ?? ?? 2
+(-
2
?? -2+2)
????
????
=
(?? -2)
?? ?? ?? (???? )
Let
????
????
=?? ?
?? 2
?? ?? ?? 2
=
????
????
? from (ii), we have
????
????
+(-
2
?? )?? =
(?? -2)
?? ?? ??
which lies linear in ?? .
I.F. =?? -?
2
?? ????
=?? -(2log ?? )
=?? log ?? -2
=?? -2
=
1
?? 2
?? (|?? ) =? (
?? -2
?? )?? ?? ·???? +?? 1
?? (
1
?? 2
) =? (
?? -2
?? )?? ?? ·
1
?? 2
???? +?? 1
=? ?? -2
?? ?? ???? -2? ?? -3
?? ?? ???? +?? 1
=?? -2
?? ?? -? (-2)?? -3
?? ?? -2? ?? -3
?? ?? ???? +?? 1
?? ?? 2
=?? -2
?? ?? +2? ?? -3
?? ?? ???? -2? ?? -3
?? ?? ???? +?? 1
?? =?? ?? +?? 1
?? 2
???? =(?? ?? +?? 1
?? 2
)????
?? =?? ?? +
1
3
?? 1
?? 3
+?? 2
? ?? =???? =?? ?? (?? ?? +
1
3
?? 1
?? 3
+?? 2
) which is the required solution.
7.5 Using the method of variation of parameters, solve the ????
(?? ?? +?? ?? +?? )?? =?? -?? ?????? (?? )
(2016 : 15 Marks)
Solution:
For complimentary function (CF)
Auxiliary equation:
?? 2
+2?? +1 =0?(?? +1)
2
=0
?? =-1,-1
C.F. =(?? 1
+?? 2
?? )?? -?? =?? 1
?? -?? +?? 2
?? ?? -??
Taking ?? -?? and ?? ·?? -?? as ?? 1
and ?? 2
For Particular Integral (PI)
Read More