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Solution of Quasilinear Partial Differential Equations of 1st Order, Lagrange's Auxiliary Equations | Mathematics Optional Notes for UPSC PDF Download

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Edurev123 
2. Solution of Quasilinear Partial 
Differential Equations of 1st Order, 
Lagrange's Auxiliary Equations and 
Charpit's Auxiliary Equations 
2.1. Find the integral surface of : 
?? ?? ?? +?? ?? ?? +?? ?? =?? 
which passes through the curve: 
???? =?? +?? ,?? =?? 
(2009 : 6 Marks) 
Solution: 
 Given ??? 2
?? +?? 2
?? +?? 2
=0 or ?? 2
?? +?? 2
?? =?? 2
(1) 
Given curve is 
???? =?? +?? ,?? =1 (2) 
Here Lagrange's auxiliary equations are 
????
?? 2
=
????
?? 2
=
????
(-?? 2
)
(3) 
Taking the first and third fractions, 
?? 2
???? +?? 2
???? =0
?????????????? ?? ?????? ???????????????????????????????????-(
1
?? )-(
1
?? )=-?? 1
 or ?
1
?? +
1
?? =?? 1
?????????????????????????????????????????????????????????????????????????????????????(4)
?? 2
???? +?? 2
???? =0
 
(
1
?? )+(
1
?? )=-?? 2
 or 
1
?? +
1
?? =?? 2
(5) 
Adding (4) and (5) 
Page 2


Edurev123 
2. Solution of Quasilinear Partial 
Differential Equations of 1st Order, 
Lagrange's Auxiliary Equations and 
Charpit's Auxiliary Equations 
2.1. Find the integral surface of : 
?? ?? ?? +?? ?? ?? +?? ?? =?? 
which passes through the curve: 
???? =?? +?? ,?? =?? 
(2009 : 6 Marks) 
Solution: 
 Given ??? 2
?? +?? 2
?? +?? 2
=0 or ?? 2
?? +?? 2
?? =?? 2
(1) 
Given curve is 
???? =?? +?? ,?? =1 (2) 
Here Lagrange's auxiliary equations are 
????
?? 2
=
????
?? 2
=
????
(-?? 2
)
(3) 
Taking the first and third fractions, 
?? 2
???? +?? 2
???? =0
?????????????? ?? ?????? ???????????????????????????????????-(
1
?? )-(
1
?? )=-?? 1
 or ?
1
?? +
1
?? =?? 1
?????????????????????????????????????????????????????????????????????????????????????(4)
?? 2
???? +?? 2
???? =0
 
(
1
?? )+(
1
?? )=-?? 2
 or 
1
?? +
1
?? =?? 2
(5) 
Adding (4) and (5) 
1
?? +
1
?? +
2
?? =?? 1
+?? 2
?? +?? ????
+
2
?? =?? 1
+?? 2
 
(?? +?? )(???? )+2?=?? 1
+?? 2
, using (2)  
?? 1
+?? 2
=3 
Substituting the values of ?? 1
 and ?? 2
 from (4) and (5) in (6), we get 
1
?? +
1
?? +
1
?? +
1
?? =3 or ???? +2???? +???? =3?????? 
2.2 Solve the PDE : 
(?? +?? ?? )
?? ?? ?? ?? +(?? ???? -?? )
?? ?? ?? ?? =?? ?? ?? +?? 
(2011: 12 marks) 
Solution: 
The given partial differential equation is 
(?? +2?? )
??? ??? +(4???? -?? )
??? ??? =2?? 2
+?? 
Putting 
??? ??? =?? and 
??? ??? =?? , 
(?? +2?? )?? +(4???? -?? )?? =2?? 2
+?? 
It is of the form of 
?? ?? +?? ?? =?? 
The Lagrange's auxiliary equations are 
????
?? =
????
?? =
????
?? or 
????
?? +2?? =
????
4???? -?? =
????
2?? 2
+?? 
Using 2?? ,-1,-1 as multipliers, each fraction in (i) is equal to 
2?????? -???? -????
2?? 2
+4???? -4???? +?? -2?? 2
-?? ?=
2?????? -???? -????
0
???????????????????????????????????????????????????????????????????????????? 2
-?? -?? ?=?? 1
 
Page 3


Edurev123 
2. Solution of Quasilinear Partial 
Differential Equations of 1st Order, 
Lagrange's Auxiliary Equations and 
Charpit's Auxiliary Equations 
2.1. Find the integral surface of : 
?? ?? ?? +?? ?? ?? +?? ?? =?? 
which passes through the curve: 
???? =?? +?? ,?? =?? 
(2009 : 6 Marks) 
Solution: 
 Given ??? 2
?? +?? 2
?? +?? 2
=0 or ?? 2
?? +?? 2
?? =?? 2
(1) 
Given curve is 
???? =?? +?? ,?? =1 (2) 
Here Lagrange's auxiliary equations are 
????
?? 2
=
????
?? 2
=
????
(-?? 2
)
(3) 
Taking the first and third fractions, 
?? 2
???? +?? 2
???? =0
?????????????? ?? ?????? ???????????????????????????????????-(
1
?? )-(
1
?? )=-?? 1
 or ?
1
?? +
1
?? =?? 1
?????????????????????????????????????????????????????????????????????????????????????(4)
?? 2
???? +?? 2
???? =0
 
(
1
?? )+(
1
?? )=-?? 2
 or 
1
?? +
1
?? =?? 2
(5) 
Adding (4) and (5) 
1
?? +
1
?? +
2
?? =?? 1
+?? 2
?? +?? ????
+
2
?? =?? 1
+?? 2
 
(?? +?? )(???? )+2?=?? 1
+?? 2
, using (2)  
?? 1
+?? 2
=3 
Substituting the values of ?? 1
 and ?? 2
 from (4) and (5) in (6), we get 
1
?? +
1
?? +
1
?? +
1
?? =3 or ???? +2???? +???? =3?????? 
2.2 Solve the PDE : 
(?? +?? ?? )
?? ?? ?? ?? +(?? ???? -?? )
?? ?? ?? ?? =?? ?? ?? +?? 
(2011: 12 marks) 
Solution: 
The given partial differential equation is 
(?? +2?? )
??? ??? +(4???? -?? )
??? ??? =2?? 2
+?? 
Putting 
??? ??? =?? and 
??? ??? =?? , 
(?? +2?? )?? +(4???? -?? )?? =2?? 2
+?? 
It is of the form of 
?? ?? +?? ?? =?? 
The Lagrange's auxiliary equations are 
????
?? =
????
?? =
????
?? or 
????
?? +2?? =
????
4???? -?? =
????
2?? 2
+?? 
Using 2?? ,-1,-1 as multipliers, each fraction in (i) is equal to 
2?????? -???? -????
2?? 2
+4???? -4???? +?? -2?? 2
-?? ?=
2?????? -???? -????
0
???????????????????????????????????????????????????????????????????????????? 2
-?? -?? ?=?? 1
 
Taking ?? ,?? ,-2?? as multipliers, we have, 
?????? +?????? -2??????
???? +2???? +4?? ?? 2
-???? -4?? 2
?? -2????
=
?????? +?????? -2??????
0
??????? -?? 2
=?? 2
????? (?? 2
-?? -?? ,???? -?? 2
)=0
 
2.3 Solve the partial differential equation ???? +???? =?? ?? . 
(2012 : 20 Marks) 
Solution: 
The given equation is 
???? +???? =32 (??) 
The Lagrange's Auxiliary equation is 
????
?? =
????
?? =
????
?? (???? ) 
Taking first two fractions of equation (ii), 
????
?? =
????
?? 
on?integrating,?????????????????????????????????????????????????log??? ?=log??? +log??? 1
?? ?=?? ?? 1
?
?? ?? =?? 1
 
Taking last two fractions of equation (ii), 
????????????????????????????????????????????
????
?? =
????
3?? ?3log??? =log??? +log??? 2
?????????????????????????????????????????
?? 3
?? =?? 2
 
So, required general solution of (i) is 
?? (?? 1
,?? 2
)?=0
?? (
?? ?? ,
?? 3
?? )?=0
 
2.4 Solve the partial differential equation 
(?? ?? +?? ?? -?? ?? )?? -?? ?????? +?? ???? =?? 
where 
Page 4


Edurev123 
2. Solution of Quasilinear Partial 
Differential Equations of 1st Order, 
Lagrange's Auxiliary Equations and 
Charpit's Auxiliary Equations 
2.1. Find the integral surface of : 
?? ?? ?? +?? ?? ?? +?? ?? =?? 
which passes through the curve: 
???? =?? +?? ,?? =?? 
(2009 : 6 Marks) 
Solution: 
 Given ??? 2
?? +?? 2
?? +?? 2
=0 or ?? 2
?? +?? 2
?? =?? 2
(1) 
Given curve is 
???? =?? +?? ,?? =1 (2) 
Here Lagrange's auxiliary equations are 
????
?? 2
=
????
?? 2
=
????
(-?? 2
)
(3) 
Taking the first and third fractions, 
?? 2
???? +?? 2
???? =0
?????????????? ?? ?????? ???????????????????????????????????-(
1
?? )-(
1
?? )=-?? 1
 or ?
1
?? +
1
?? =?? 1
?????????????????????????????????????????????????????????????????????????????????????(4)
?? 2
???? +?? 2
???? =0
 
(
1
?? )+(
1
?? )=-?? 2
 or 
1
?? +
1
?? =?? 2
(5) 
Adding (4) and (5) 
1
?? +
1
?? +
2
?? =?? 1
+?? 2
?? +?? ????
+
2
?? =?? 1
+?? 2
 
(?? +?? )(???? )+2?=?? 1
+?? 2
, using (2)  
?? 1
+?? 2
=3 
Substituting the values of ?? 1
 and ?? 2
 from (4) and (5) in (6), we get 
1
?? +
1
?? +
1
?? +
1
?? =3 or ???? +2???? +???? =3?????? 
2.2 Solve the PDE : 
(?? +?? ?? )
?? ?? ?? ?? +(?? ???? -?? )
?? ?? ?? ?? =?? ?? ?? +?? 
(2011: 12 marks) 
Solution: 
The given partial differential equation is 
(?? +2?? )
??? ??? +(4???? -?? )
??? ??? =2?? 2
+?? 
Putting 
??? ??? =?? and 
??? ??? =?? , 
(?? +2?? )?? +(4???? -?? )?? =2?? 2
+?? 
It is of the form of 
?? ?? +?? ?? =?? 
The Lagrange's auxiliary equations are 
????
?? =
????
?? =
????
?? or 
????
?? +2?? =
????
4???? -?? =
????
2?? 2
+?? 
Using 2?? ,-1,-1 as multipliers, each fraction in (i) is equal to 
2?????? -???? -????
2?? 2
+4???? -4???? +?? -2?? 2
-?? ?=
2?????? -???? -????
0
???????????????????????????????????????????????????????????????????????????? 2
-?? -?? ?=?? 1
 
Taking ?? ,?? ,-2?? as multipliers, we have, 
?????? +?????? -2??????
???? +2???? +4?? ?? 2
-???? -4?? 2
?? -2????
=
?????? +?????? -2??????
0
??????? -?? 2
=?? 2
????? (?? 2
-?? -?? ,???? -?? 2
)=0
 
2.3 Solve the partial differential equation ???? +???? =?? ?? . 
(2012 : 20 Marks) 
Solution: 
The given equation is 
???? +???? =32 (??) 
The Lagrange's Auxiliary equation is 
????
?? =
????
?? =
????
?? (???? ) 
Taking first two fractions of equation (ii), 
????
?? =
????
?? 
on?integrating,?????????????????????????????????????????????????log??? ?=log??? +log??? 1
?? ?=?? ?? 1
?
?? ?? =?? 1
 
Taking last two fractions of equation (ii), 
????????????????????????????????????????????
????
?? =
????
3?? ?3log??? =log??? +log??? 2
?????????????????????????????????????????
?? 3
?? =?? 2
 
So, required general solution of (i) is 
?? (?? 1
,?? 2
)?=0
?? (
?? ?? ,
?? 3
?? )?=0
 
2.4 Solve the partial differential equation 
(?? ?? +?? ?? -?? ?? )?? -?? ?????? +?? ???? =?? 
where 
?? =
?? ?? ?? ?? ,?? =
?? ?? ?? ?? 
(2015 : 10 Marks) 
Solution: 
Given equation is : 
(?? 2
+?? 2
-?? 2
)?? -2?????? +2???? =0
(?? 2
+?? 2
-?? 2
)?? -2?????? =-2????
 
Lagrange's auxiliary equation are : 
????
?? 2
+?? 2
-?? 2
=
????
-2????
=
????
-2????
 
Consider 
????
-2????
=
????
-2×?? ?
????
?? =
????
?? 
Integrating it, 
? log??? =log??? +log?(?? ), where ?? is a constant: 
? log?(
?? ?? ) =log??? ? ?? =????
(??) 
Also, using ?? ,?? and ?? as multipliers, we get 
???
?????? +?????? +??????
?? (?? 2
+?? 2
-?? 2
)-?? (2?? 2
)-?? (2?? 2
)
=
????
-2????
????????????????
?????? +?????? +??????
?? (?? 2
+?? 2
-?? 2
-2?? 2
-2?? 2
)
=
????
????
???????????????????????????????
2?????? +2?????? +2??????
?? 2
+?? 2
+?? 2
=
????
?? 
Integrating both sides, we get 
log?(?? 2
+?? 2
+?? 2
)=log??? +log??? ; where ?? is a constant.  
???? 2
+?? 2
+?? 2
= by (???? ) 
From (i) and (ii), solution is given by 
?? (?? -???? ,?? 2
+?? 2
+?? 2
-???? )=0 
Page 5


Edurev123 
2. Solution of Quasilinear Partial 
Differential Equations of 1st Order, 
Lagrange's Auxiliary Equations and 
Charpit's Auxiliary Equations 
2.1. Find the integral surface of : 
?? ?? ?? +?? ?? ?? +?? ?? =?? 
which passes through the curve: 
???? =?? +?? ,?? =?? 
(2009 : 6 Marks) 
Solution: 
 Given ??? 2
?? +?? 2
?? +?? 2
=0 or ?? 2
?? +?? 2
?? =?? 2
(1) 
Given curve is 
???? =?? +?? ,?? =1 (2) 
Here Lagrange's auxiliary equations are 
????
?? 2
=
????
?? 2
=
????
(-?? 2
)
(3) 
Taking the first and third fractions, 
?? 2
???? +?? 2
???? =0
?????????????? ?? ?????? ???????????????????????????????????-(
1
?? )-(
1
?? )=-?? 1
 or ?
1
?? +
1
?? =?? 1
?????????????????????????????????????????????????????????????????????????????????????(4)
?? 2
???? +?? 2
???? =0
 
(
1
?? )+(
1
?? )=-?? 2
 or 
1
?? +
1
?? =?? 2
(5) 
Adding (4) and (5) 
1
?? +
1
?? +
2
?? =?? 1
+?? 2
?? +?? ????
+
2
?? =?? 1
+?? 2
 
(?? +?? )(???? )+2?=?? 1
+?? 2
, using (2)  
?? 1
+?? 2
=3 
Substituting the values of ?? 1
 and ?? 2
 from (4) and (5) in (6), we get 
1
?? +
1
?? +
1
?? +
1
?? =3 or ???? +2???? +???? =3?????? 
2.2 Solve the PDE : 
(?? +?? ?? )
?? ?? ?? ?? +(?? ???? -?? )
?? ?? ?? ?? =?? ?? ?? +?? 
(2011: 12 marks) 
Solution: 
The given partial differential equation is 
(?? +2?? )
??? ??? +(4???? -?? )
??? ??? =2?? 2
+?? 
Putting 
??? ??? =?? and 
??? ??? =?? , 
(?? +2?? )?? +(4???? -?? )?? =2?? 2
+?? 
It is of the form of 
?? ?? +?? ?? =?? 
The Lagrange's auxiliary equations are 
????
?? =
????
?? =
????
?? or 
????
?? +2?? =
????
4???? -?? =
????
2?? 2
+?? 
Using 2?? ,-1,-1 as multipliers, each fraction in (i) is equal to 
2?????? -???? -????
2?? 2
+4???? -4???? +?? -2?? 2
-?? ?=
2?????? -???? -????
0
???????????????????????????????????????????????????????????????????????????? 2
-?? -?? ?=?? 1
 
Taking ?? ,?? ,-2?? as multipliers, we have, 
?????? +?????? -2??????
???? +2???? +4?? ?? 2
-???? -4?? 2
?? -2????
=
?????? +?????? -2??????
0
??????? -?? 2
=?? 2
????? (?? 2
-?? -?? ,???? -?? 2
)=0
 
2.3 Solve the partial differential equation ???? +???? =?? ?? . 
(2012 : 20 Marks) 
Solution: 
The given equation is 
???? +???? =32 (??) 
The Lagrange's Auxiliary equation is 
????
?? =
????
?? =
????
?? (???? ) 
Taking first two fractions of equation (ii), 
????
?? =
????
?? 
on?integrating,?????????????????????????????????????????????????log??? ?=log??? +log??? 1
?? ?=?? ?? 1
?
?? ?? =?? 1
 
Taking last two fractions of equation (ii), 
????????????????????????????????????????????
????
?? =
????
3?? ?3log??? =log??? +log??? 2
?????????????????????????????????????????
?? 3
?? =?? 2
 
So, required general solution of (i) is 
?? (?? 1
,?? 2
)?=0
?? (
?? ?? ,
?? 3
?? )?=0
 
2.4 Solve the partial differential equation 
(?? ?? +?? ?? -?? ?? )?? -?? ?????? +?? ???? =?? 
where 
?? =
?? ?? ?? ?? ,?? =
?? ?? ?? ?? 
(2015 : 10 Marks) 
Solution: 
Given equation is : 
(?? 2
+?? 2
-?? 2
)?? -2?????? +2???? =0
(?? 2
+?? 2
-?? 2
)?? -2?????? =-2????
 
Lagrange's auxiliary equation are : 
????
?? 2
+?? 2
-?? 2
=
????
-2????
=
????
-2????
 
Consider 
????
-2????
=
????
-2×?? ?
????
?? =
????
?? 
Integrating it, 
? log??? =log??? +log?(?? ), where ?? is a constant: 
? log?(
?? ?? ) =log??? ? ?? =????
(??) 
Also, using ?? ,?? and ?? as multipliers, we get 
???
?????? +?????? +??????
?? (?? 2
+?? 2
-?? 2
)-?? (2?? 2
)-?? (2?? 2
)
=
????
-2????
????????????????
?????? +?????? +??????
?? (?? 2
+?? 2
-?? 2
-2?? 2
-2?? 2
)
=
????
????
???????????????????????????????
2?????? +2?????? +2??????
?? 2
+?? 2
+?? 2
=
????
?? 
Integrating both sides, we get 
log?(?? 2
+?? 2
+?? 2
)=log??? +log??? ; where ?? is a constant.  
???? 2
+?? 2
+?? 2
= by (???? ) 
From (i) and (ii), solution is given by 
?? (?? -???? ,?? 2
+?? 2
+?? 2
-???? )=0 
2.5 Solve for the general solution ?? ?????? ?(?? +?? )+?? ?????? ?(?? +?? )=?? , where ?? =
?? ?? ?? ?? and 
?? =
?? ?? ?? ?? . 
(2015 : 15 Marks) 
Solution: 
Given equation is : 
?? cos?(?? +?? )+?? sin?(?? +?? )=?? 
We first write Lagrange's auxiliary equation for given equation 
i.e., 
????
cos?(?? +?? )
=
????
sin?(?? +?? )
=
????
?? 
Consider 
???? +????
cos?(?? +?? )+sin?(?? +?? )
=
????
?? 
?
???? +????
v2×
1
v2
{cos?(?? +?? )+sin?(?? +?? )}
=
????
?? 
?
???? +????
v2{
cos?(?? +?? )
v2
+
sin?(?? +?? )
v2
}
=
????
?? 
?
???? +????
v2{cos?(?? +?? )sin?(
?? 4
)+sin?(?? +?? )cos?(
?? 4
)}
=
????
?? 
?
?? (?? +?? +
?? 4
)
v2sin?(?? +?? +
?? 4
)
=
????
?? ??cosec(?? +?? +
?? 4
)?? (?? +?? +
?? 4
)=
????
?? 
Integrating both sides, we get 
??????????????????????????-log?[cosec?(?? +?? +
?? 4
)+cot?(?? +?? +
?? 4
)]=
????
?? log??? +log??? ; 
where ?? is a constant. 
?log??? +log?{cosec?(?? +?? +
?? 4
)+cot?(?? +?? +
?? 4
)}+log??? =0
? ????????????????????????????????????????? (cosec?(?? +?? +
?? 4
)+cot?(?? +?? +
?? 4
))=
1
?? 
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