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Linear P.D.E. of Second Order with Constant Coefficient | Mathematics Optional Notes for UPSC PDF Download

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 Page 2


 At ?? =0, ?? 0
=
-?? 2
4
?
-?? 2
4
=
-?? 2
4
+?? 5
??? 5
=0
?? =
-?? 2
4
?? 2?? (?????? ) 
From (x), (xi) and (xii), characteristics are 
?? =?? (2-?? ?? )
?? =v2?? (1-?? ?? )
?? =
-?? 2
4
?? 2?? 
To find the equation of integral surface, we eliminate ?? and ?? ?? from (x) and (xi) and put 
their values in (xii). 
and 
?? ?? ?=
v2(?? -v2?? )
v2?? -?? ?? ?=
v2?? -?? v2
 
Using these values of ?? and ?? ?? , we get 
?? =
-?? 2
4
?? 2?? =-
(v2?? -?? )
2
2
×
(v2?? -2?? )
2
(v2?? -?? )
2
???? =-
2
?? (?? -v2?? )
2
???? =-(?? -v2?? )
2
, which is the required surface. 
 
Edurev123 
4. Linear P.D.E. of Second Order with 
Constant Coefficient 
4.1 Solve : 
(?? ?? -?? ?? '
-?? ?? ?? )?? =(?? ?? ?? +???? -?? ?? )?????? ????? -?????? ????? 
where ?? and ?? '
 represent 
?? ?? ?? and 
?? ?? ?? . 
(2009 : 15 Marks) 
Page 3


 At ?? =0, ?? 0
=
-?? 2
4
?
-?? 2
4
=
-?? 2
4
+?? 5
??? 5
=0
?? =
-?? 2
4
?? 2?? (?????? ) 
From (x), (xi) and (xii), characteristics are 
?? =?? (2-?? ?? )
?? =v2?? (1-?? ?? )
?? =
-?? 2
4
?? 2?? 
To find the equation of integral surface, we eliminate ?? and ?? ?? from (x) and (xi) and put 
their values in (xii). 
and 
?? ?? ?=
v2(?? -v2?? )
v2?? -?? ?? ?=
v2?? -?? v2
 
Using these values of ?? and ?? ?? , we get 
?? =
-?? 2
4
?? 2?? =-
(v2?? -?? )
2
2
×
(v2?? -2?? )
2
(v2?? -?? )
2
???? =-
2
?? (?? -v2?? )
2
???? =-(?? -v2?? )
2
, which is the required surface. 
 
Edurev123 
4. Linear P.D.E. of Second Order with 
Constant Coefficient 
4.1 Solve : 
(?? ?? -?? ?? '
-?? ?? ?? )?? =(?? ?? ?? +???? -?? ?? )?????? ????? -?????? ????? 
where ?? and ?? '
 represent 
?? ?? ?? and 
?? ?? ?? . 
(2009 : 15 Marks) 
Solution: 
Clearly the Auxiliary equation is 
?? 2
-?? -2 =0
? ?? 2
-2?? +?? -2 =0
? ?? (?? -2)+1(?? -2) =0
? ?? =-1,2
?? ?? =?? 1
(?? +2?? )+?? 2
(?? -?? )
 
where ?? ?
1
 and ?? 2
 are Arbitrary function. 
Particular Integral 
=
1
(?? +?? '
)(?? -2?? '
)
[(2?? 2
+???? -2?? 2
)sin????? -cos????? ] 
Putting ?? -?? =?? we get ?? =?? +?? 
?=
1
(?? -2?? '
)
??[2?? 2
+?? (?? +?? )-2(?? +?? )
2
sin??? (?? +?? )-cos??? (?? +?? )]????
?=
1
(?? -2?? '
)
??[2?? 2
+(?? +?? )[?? -2?? -2?? |sin?(?? 2
+???? )-cos?(?? '
+???? )]????
?=
1
(?? -2?? '
)
??[2?? 2
+(?? +?? )(-?? -2?? )sin?(?? 2
+???? )-cos?(?? 2
+???? )]????
=
1
(?? -2?? '
)
??[2?? +?? )(?? -?? )sin?(?? 2
+???? )-cos?(?? 2
+???? )]????
?=
1
(?? -2?? '
)
??-(2?? +?? )(?? -?? )
cos?(?? 2
+???? )
(2?? +?? )
+??cos?(?? 2
+???? )???? -??cos?(?? 2
+???? )????
=
1
(?? -2?? '
)
(?? -2?? )cos????? ](?? '
-2?? -2?? )cos??? (?? '
-2?? )???? -?? =?? +2?? ????(?? '
-4?? )cos?(?? '
?? -2?? 2
)????
??
[sin?(?? '
?? -2?? 2
)]
(?? '
-4?? )
(?? '
-4?? )
?=sin?[(?? +2?? )?? -2?? 2
]
?=sin?????
 
Clearly, the required solution is ?? =?? 1
+?? 2
 
4.2 Solve the PDE: 
(?? ?? -?? '
)(?? -?? ?? '
)?? =?? ?? ?? +?? +???? 
(2010 : 12 Marks) 
Solution: 
Page 4


 At ?? =0, ?? 0
=
-?? 2
4
?
-?? 2
4
=
-?? 2
4
+?? 5
??? 5
=0
?? =
-?? 2
4
?? 2?? (?????? ) 
From (x), (xi) and (xii), characteristics are 
?? =?? (2-?? ?? )
?? =v2?? (1-?? ?? )
?? =
-?? 2
4
?? 2?? 
To find the equation of integral surface, we eliminate ?? and ?? ?? from (x) and (xi) and put 
their values in (xii). 
and 
?? ?? ?=
v2(?? -v2?? )
v2?? -?? ?? ?=
v2?? -?? v2
 
Using these values of ?? and ?? ?? , we get 
?? =
-?? 2
4
?? 2?? =-
(v2?? -?? )
2
2
×
(v2?? -2?? )
2
(v2?? -?? )
2
???? =-
2
?? (?? -v2?? )
2
???? =-(?? -v2?? )
2
, which is the required surface. 
 
Edurev123 
4. Linear P.D.E. of Second Order with 
Constant Coefficient 
4.1 Solve : 
(?? ?? -?? ?? '
-?? ?? ?? )?? =(?? ?? ?? +???? -?? ?? )?????? ????? -?????? ????? 
where ?? and ?? '
 represent 
?? ?? ?? and 
?? ?? ?? . 
(2009 : 15 Marks) 
Solution: 
Clearly the Auxiliary equation is 
?? 2
-?? -2 =0
? ?? 2
-2?? +?? -2 =0
? ?? (?? -2)+1(?? -2) =0
? ?? =-1,2
?? ?? =?? 1
(?? +2?? )+?? 2
(?? -?? )
 
where ?? ?
1
 and ?? 2
 are Arbitrary function. 
Particular Integral 
=
1
(?? +?? '
)(?? -2?? '
)
[(2?? 2
+???? -2?? 2
)sin????? -cos????? ] 
Putting ?? -?? =?? we get ?? =?? +?? 
?=
1
(?? -2?? '
)
??[2?? 2
+?? (?? +?? )-2(?? +?? )
2
sin??? (?? +?? )-cos??? (?? +?? )]????
?=
1
(?? -2?? '
)
??[2?? 2
+(?? +?? )[?? -2?? -2?? |sin?(?? 2
+???? )-cos?(?? '
+???? )]????
?=
1
(?? -2?? '
)
??[2?? 2
+(?? +?? )(-?? -2?? )sin?(?? 2
+???? )-cos?(?? 2
+???? )]????
=
1
(?? -2?? '
)
??[2?? +?? )(?? -?? )sin?(?? 2
+???? )-cos?(?? 2
+???? )]????
?=
1
(?? -2?? '
)
??-(2?? +?? )(?? -?? )
cos?(?? 2
+???? )
(2?? +?? )
+??cos?(?? 2
+???? )???? -??cos?(?? 2
+???? )????
=
1
(?? -2?? '
)
(?? -2?? )cos????? ](?? '
-2?? -2?? )cos??? (?? '
-2?? )???? -?? =?? +2?? ????(?? '
-4?? )cos?(?? '
?? -2?? 2
)????
??
[sin?(?? '
?? -2?? 2
)]
(?? '
-4?? )
(?? '
-4?? )
?=sin?[(?? +2?? )?? -2?? 2
]
?=sin?????
 
Clearly, the required solution is ?? =?? 1
+?? 2
 
4.2 Solve the PDE: 
(?? ?? -?? '
)(?? -?? ?? '
)?? =?? ?? ?? +?? +???? 
(2010 : 12 Marks) 
Solution: 
Given, the equation is 
(?? 2
-?? '
)(?? -2?? '
)?? =?? 2?? +?? +???? 
Complementary Function : 
The auxiliary equation is : 
(?? 2
-1)(?? -2) =0
? ?? =±1
? Solution is ?? ?? =?? 1
 
Particular Integral : 
?? ?? =
1
(?? 2
-?? '
)(?? -2?? '
)
?? 2?? +?? +
1
(?? 2
-?? '
)(?? -2?? '
)
????
? ?? ?? =
1
(2
2
-1)(?? -2?? '
)
?? 2?? +?? +
1
(?? 2
-?? '
)·?? ×
1
(1-
2?? '
?? )
????
? ?? ?? =
1
3
1
(?? -2?? '
)
?? 2?? +?? +
1
(?? 2
-?? '
)
·
1
?? (1+
2?? '
?? )????
?? ?? ?? =
1
3
?? 1·1!
?? 2?? +?? +
1
(?? 2
-?? '
)
·
1
?? (???? +?? 2
)
?? ?? ?? =
?? 3
?? 2?? +?? +
1
(?? 2
-?? '
)
×(
?? 2
?? 2
+
?? 3
3
)
 
? ?? ?? =
?? 3
?? 2?? +?? +
1
?? 2
×(1-
?? '
?? 2
)
-1
(?? 2
?? +
?? 3
3
)
? ?? ?? =
?? 3
?? 2?? +?? +
1
?? 2
×(1+
?? '
?? 2
)(?? 2
?? +
?? 3
3
)
? ?? ?? =
?? 3
?? 2?? +?? +
1
?? 2
×(?? 2
?? +
?? 3
3
+
?? 4
12
)
? ?? ?? =
?? 3
?? 2?? +?? +
1
?? (
?? 3
?? 3
+
?? 4
12
+
?? 5
60
)
? ?? ?? =
?? 3
?? 2?? +?? +
?? 4
?? 12
+
?? 5
60
+
?? 6
360
 Now, ?? =?? ?? +?? ?? ? ?? =?? 1
(?? +?? )+?? 2
(?? -?? )+?? 3
(?? +2?? )+
?? 3
?? 2?? +?? +
?? 4
?? 12
+
?? 5
60
+
?? 6
360
 
4.3 Solve the PDE 
(?? ?? -?? ?? +?? +?? ?? '
-?? )?? =?? (?? -?? )
-?? ?? ?? 
(2011 : 12 Marks) 
Page 5


 At ?? =0, ?? 0
=
-?? 2
4
?
-?? 2
4
=
-?? 2
4
+?? 5
??? 5
=0
?? =
-?? 2
4
?? 2?? (?????? ) 
From (x), (xi) and (xii), characteristics are 
?? =?? (2-?? ?? )
?? =v2?? (1-?? ?? )
?? =
-?? 2
4
?? 2?? 
To find the equation of integral surface, we eliminate ?? and ?? ?? from (x) and (xi) and put 
their values in (xii). 
and 
?? ?? ?=
v2(?? -v2?? )
v2?? -?? ?? ?=
v2?? -?? v2
 
Using these values of ?? and ?? ?? , we get 
?? =
-?? 2
4
?? 2?? =-
(v2?? -?? )
2
2
×
(v2?? -2?? )
2
(v2?? -?? )
2
???? =-
2
?? (?? -v2?? )
2
???? =-(?? -v2?? )
2
, which is the required surface. 
 
Edurev123 
4. Linear P.D.E. of Second Order with 
Constant Coefficient 
4.1 Solve : 
(?? ?? -?? ?? '
-?? ?? ?? )?? =(?? ?? ?? +???? -?? ?? )?????? ????? -?????? ????? 
where ?? and ?? '
 represent 
?? ?? ?? and 
?? ?? ?? . 
(2009 : 15 Marks) 
Solution: 
Clearly the Auxiliary equation is 
?? 2
-?? -2 =0
? ?? 2
-2?? +?? -2 =0
? ?? (?? -2)+1(?? -2) =0
? ?? =-1,2
?? ?? =?? 1
(?? +2?? )+?? 2
(?? -?? )
 
where ?? ?
1
 and ?? 2
 are Arbitrary function. 
Particular Integral 
=
1
(?? +?? '
)(?? -2?? '
)
[(2?? 2
+???? -2?? 2
)sin????? -cos????? ] 
Putting ?? -?? =?? we get ?? =?? +?? 
?=
1
(?? -2?? '
)
??[2?? 2
+?? (?? +?? )-2(?? +?? )
2
sin??? (?? +?? )-cos??? (?? +?? )]????
?=
1
(?? -2?? '
)
??[2?? 2
+(?? +?? )[?? -2?? -2?? |sin?(?? 2
+???? )-cos?(?? '
+???? )]????
?=
1
(?? -2?? '
)
??[2?? 2
+(?? +?? )(-?? -2?? )sin?(?? 2
+???? )-cos?(?? 2
+???? )]????
=
1
(?? -2?? '
)
??[2?? +?? )(?? -?? )sin?(?? 2
+???? )-cos?(?? 2
+???? )]????
?=
1
(?? -2?? '
)
??-(2?? +?? )(?? -?? )
cos?(?? 2
+???? )
(2?? +?? )
+??cos?(?? 2
+???? )???? -??cos?(?? 2
+???? )????
=
1
(?? -2?? '
)
(?? -2?? )cos????? ](?? '
-2?? -2?? )cos??? (?? '
-2?? )???? -?? =?? +2?? ????(?? '
-4?? )cos?(?? '
?? -2?? 2
)????
??
[sin?(?? '
?? -2?? 2
)]
(?? '
-4?? )
(?? '
-4?? )
?=sin?[(?? +2?? )?? -2?? 2
]
?=sin?????
 
Clearly, the required solution is ?? =?? 1
+?? 2
 
4.2 Solve the PDE: 
(?? ?? -?? '
)(?? -?? ?? '
)?? =?? ?? ?? +?? +???? 
(2010 : 12 Marks) 
Solution: 
Given, the equation is 
(?? 2
-?? '
)(?? -2?? '
)?? =?? 2?? +?? +???? 
Complementary Function : 
The auxiliary equation is : 
(?? 2
-1)(?? -2) =0
? ?? =±1
? Solution is ?? ?? =?? 1
 
Particular Integral : 
?? ?? =
1
(?? 2
-?? '
)(?? -2?? '
)
?? 2?? +?? +
1
(?? 2
-?? '
)(?? -2?? '
)
????
? ?? ?? =
1
(2
2
-1)(?? -2?? '
)
?? 2?? +?? +
1
(?? 2
-?? '
)·?? ×
1
(1-
2?? '
?? )
????
? ?? ?? =
1
3
1
(?? -2?? '
)
?? 2?? +?? +
1
(?? 2
-?? '
)
·
1
?? (1+
2?? '
?? )????
?? ?? ?? =
1
3
?? 1·1!
?? 2?? +?? +
1
(?? 2
-?? '
)
·
1
?? (???? +?? 2
)
?? ?? ?? =
?? 3
?? 2?? +?? +
1
(?? 2
-?? '
)
×(
?? 2
?? 2
+
?? 3
3
)
 
? ?? ?? =
?? 3
?? 2?? +?? +
1
?? 2
×(1-
?? '
?? 2
)
-1
(?? 2
?? +
?? 3
3
)
? ?? ?? =
?? 3
?? 2?? +?? +
1
?? 2
×(1+
?? '
?? 2
)(?? 2
?? +
?? 3
3
)
? ?? ?? =
?? 3
?? 2?? +?? +
1
?? 2
×(?? 2
?? +
?? 3
3
+
?? 4
12
)
? ?? ?? =
?? 3
?? 2?? +?? +
1
?? (
?? 3
?? 3
+
?? 4
12
+
?? 5
60
)
? ?? ?? =
?? 3
?? 2?? +?? +
?? 4
?? 12
+
?? 5
60
+
?? 6
360
 Now, ?? =?? ?? +?? ?? ? ?? =?? 1
(?? +?? )+?? 2
(?? -?? )+?? 3
(?? +2?? )+
?? 3
?? 2?? +?? +
?? 4
?? 12
+
?? 5
60
+
?? 6
360
 
4.3 Solve the PDE 
(?? ?? -?? ?? +?? +?? ?? '
-?? )?? =?? (?? -?? )
-?? ?? ?? 
(2011 : 12 Marks) 
Solution: 
The given partial differential equation is 
(?? 2
-?? 2
+?? +3?? -2)?? ?=?? ?? -?? -?? 2
?? 
? the compiementary function of (??) is 
?? ?? =?? -2?? ?? 1
(?? +?? )+?? ?? ?? 2
(?? -?? ) (???? ) 
The particular integral of (i) is 
?? ?? =
1
(?? -?? '
+2)(?? +?? '
-1)
(?? ?? -?? -?? 2
?? ) 
=
1
(?? -?? '
+2)(?? +?? '
-1)
?? ?? -?? -
1
(?? -?? '
+2)(?? +?? '
-1)
·?? 2
?? 
=
1
(1+1+2)(1-1-1)
?? ?? -?? -
1
?? 2
-?? 2
+?? +3?? '
-2
·?? 2
?? 
=
-1
4
?? ?? -?? +
1
2[1-
?? 2
-?? 2
+?? +3?? 2
]
·?? 2
?? 
=
-1
4
?? ?? -?? +
1
2
[1+(
?? 2
-?? 2
+?? +3?? '
2
)+(
?? 2
-?? 2
+?? +3?? '
2
)
2
+?..]?? 2
?? 
=
-1
4
?? ?? -?? +
1
2
[1+
?? 2
+
3?? '
2
+(
1
2
+
1
4
)?? 2
+(
9
4
-
1
2
)?? '2
+
3
2
?? ?? '
+
21
8
?? 2
?? '
-
23
8
?? ?? '2
…..]?? 2
?? 
=
-1
4
?? ?? -?? +
1
2
[?? 2
?? +???? +
3
2
?? 2
+
3
2
?? +3?? +
21
4
] 
??? =?? ?? +?? ?? =?? -2?? ?? 1
(?? +?? )+?? ?? ?? 2
(?? +?? )-
1
4
?? ?? -?? +
1
2
[?? 2
?? +???? +
3
2
?? 2
+
3
2
?? +3?? +
21
4
] 
4.4 Solve the partial differential equation: 
(?? -?? ?? '
)(?? -?? '
)
?? ?? =?? ?? +?? 
(2011 : 12 Marks) 
Solution: 
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