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Edurev123 
Partial Differential Equations 
1. Formulation of P.D.E. 
1.1 Show that the differential equation of all cones which have their vertex at the 
origin is ???? +???? =?? . Verify that this equation is satisfied by the surface ???? +?? ?? +
???? =?? . 
(2009 : 12 Marks) 
Solution: 
The equation cone having vertex at origin 
?? ?? 2
+?? ?? 2
+?? ?? 2
+2h???? +2?????? +2?????? =0 (1) 
where ???,?? ,?? ,?? ,?? ,h are parameters. 
Differentiating w.r.t. ?? and ?? , we get 
2???? +2h?? +2???? +2?????? +2?????? +2?????? ?=0
2???? +2?????? +2h?? +2?????? +2???? +2?????? ?=0
???? +h?? +???? +?? (???? +???? +???? )?=0×?? ???? +h?? +???? +?? (???? +???? +???? )?=0×?? ?? ?? 2
+h???? +?????? +?? (?? ?? 2
+?????? +?????? )?=0
?? ?? 2
+h???? +?????? +?? (?????? +?? ?? 2
+?????? )?=0
 
On adding, 
???????????????? ?? 2
+?? ?? 2
+2h???? +?????? +?????? +???? +???? [???? +???? +???? ]=0?????????
???????????????????????????????-(?? ?? 2
+?????? +?????? )+(???? +???? +???? )(???? +???? )=0?????????
????????????????????????????????????????????????????????????????????(???? +???? +???? )(???? +???? -?? )=0?????????
 
Clearly, ???? +???? -?? =0 is required differential equation. 
Given surface is ???? +???? +???? =0 
Differentiating (?
*
) w.r.t. ?? and ?? , we get 
???? +?? +???? +?? =0 (2)
?? +???? +???? +?? =0 (3)
 
So, we get 
Page 2


Edurev123 
Partial Differential Equations 
1. Formulation of P.D.E. 
1.1 Show that the differential equation of all cones which have their vertex at the 
origin is ???? +???? =?? . Verify that this equation is satisfied by the surface ???? +?? ?? +
???? =?? . 
(2009 : 12 Marks) 
Solution: 
The equation cone having vertex at origin 
?? ?? 2
+?? ?? 2
+?? ?? 2
+2h???? +2?????? +2?????? =0 (1) 
where ???,?? ,?? ,?? ,?? ,h are parameters. 
Differentiating w.r.t. ?? and ?? , we get 
2???? +2h?? +2???? +2?????? +2?????? +2?????? ?=0
2???? +2?????? +2h?? +2?????? +2???? +2?????? ?=0
???? +h?? +???? +?? (???? +???? +???? )?=0×?? ???? +h?? +???? +?? (???? +???? +???? )?=0×?? ?? ?? 2
+h???? +?????? +?? (?? ?? 2
+?????? +?????? )?=0
?? ?? 2
+h???? +?????? +?? (?????? +?? ?? 2
+?????? )?=0
 
On adding, 
???????????????? ?? 2
+?? ?? 2
+2h???? +?????? +?????? +???? +???? [???? +???? +???? ]=0?????????
???????????????????????????????-(?? ?? 2
+?????? +?????? )+(???? +???? +???? )(???? +???? )=0?????????
????????????????????????????????????????????????????????????????????(???? +???? +???? )(???? +???? -?? )=0?????????
 
Clearly, ???? +???? -?? =0 is required differential equation. 
Given surface is ???? +???? +???? =0 
Differentiating (?
*
) w.r.t. ?? and ?? , we get 
???? +?? +???? +?? =0 (2)
?? +???? +???? +?? =0 (3)
 
So, we get 
?? ?=
-(?? +?? )
(?? +?? )
,?? =
-(?? +?? )
(?? +?? )
???? +???? -?? ?=
-(?? +?? )?? (?? +?? )
-
(?? +?? )
(?? +?? )
?? -?? ?=
-(?? +?? )?? -(?? +?? )?? -?? (?? +?? )
(?? +?? )
?=
-???? -???? -???? -???? -???? -????
(?? +?? )
?=
-2(???? +???? +???? )
?? +?? =
-20
?? +?? =0
 
1.2 From the partial differential equation by eliminating the arbitrary function ?? 
given by: 
?? (?? ?? +?? ?? ,?? -???? )=?? 
Solution: 
The function is 
???????????????????????????????????????????????????????????????????? =???? +?? (?? 2
+?? 2
)??????????????????????????????????????????????????????????????(1) 
Now differentiating partially (1) w.r.t. ?? we get 
??? ??? =?? +?? '
(?? 2
+?? 2
)2?? 
So, 
?? -?? 2?? =?? '
(?? 2
+?? 2
) (2) 
Now, differentiating partially (1) w.r.t. ?? , we get 
??? ??? =?? +?? (?? 2
+?? 2
)·2?? 
So, 
?? -?? 2?? =?? '
(?? 2
+?? 2
) (3)
 
Equating (2) and (3), we get 
?? -?? 2?? =
?? -?? 2?? 
So, ???? -???? =?? 2
-?? 2
 is linear PDE. 
1.3 Find the surface satisfying the P.D.E. (?? ?? -?? ?? ?? '
+?? ?? )?? =?? and the 
conditions that ???? =?? ?? when ?? =?? and ???? =?? ?? when ?? =?? . 
Page 3


Edurev123 
Partial Differential Equations 
1. Formulation of P.D.E. 
1.1 Show that the differential equation of all cones which have their vertex at the 
origin is ???? +???? =?? . Verify that this equation is satisfied by the surface ???? +?? ?? +
???? =?? . 
(2009 : 12 Marks) 
Solution: 
The equation cone having vertex at origin 
?? ?? 2
+?? ?? 2
+?? ?? 2
+2h???? +2?????? +2?????? =0 (1) 
where ???,?? ,?? ,?? ,?? ,h are parameters. 
Differentiating w.r.t. ?? and ?? , we get 
2???? +2h?? +2???? +2?????? +2?????? +2?????? ?=0
2???? +2?????? +2h?? +2?????? +2???? +2?????? ?=0
???? +h?? +???? +?? (???? +???? +???? )?=0×?? ???? +h?? +???? +?? (???? +???? +???? )?=0×?? ?? ?? 2
+h???? +?????? +?? (?? ?? 2
+?????? +?????? )?=0
?? ?? 2
+h???? +?????? +?? (?????? +?? ?? 2
+?????? )?=0
 
On adding, 
???????????????? ?? 2
+?? ?? 2
+2h???? +?????? +?????? +???? +???? [???? +???? +???? ]=0?????????
???????????????????????????????-(?? ?? 2
+?????? +?????? )+(???? +???? +???? )(???? +???? )=0?????????
????????????????????????????????????????????????????????????????????(???? +???? +???? )(???? +???? -?? )=0?????????
 
Clearly, ???? +???? -?? =0 is required differential equation. 
Given surface is ???? +???? +???? =0 
Differentiating (?
*
) w.r.t. ?? and ?? , we get 
???? +?? +???? +?? =0 (2)
?? +???? +???? +?? =0 (3)
 
So, we get 
?? ?=
-(?? +?? )
(?? +?? )
,?? =
-(?? +?? )
(?? +?? )
???? +???? -?? ?=
-(?? +?? )?? (?? +?? )
-
(?? +?? )
(?? +?? )
?? -?? ?=
-(?? +?? )?? -(?? +?? )?? -?? (?? +?? )
(?? +?? )
?=
-???? -???? -???? -???? -???? -????
(?? +?? )
?=
-2(???? +???? +???? )
?? +?? =
-20
?? +?? =0
 
1.2 From the partial differential equation by eliminating the arbitrary function ?? 
given by: 
?? (?? ?? +?? ?? ,?? -???? )=?? 
Solution: 
The function is 
???????????????????????????????????????????????????????????????????? =???? +?? (?? 2
+?? 2
)??????????????????????????????????????????????????????????????(1) 
Now differentiating partially (1) w.r.t. ?? we get 
??? ??? =?? +?? '
(?? 2
+?? 2
)2?? 
So, 
?? -?? 2?? =?? '
(?? 2
+?? 2
) (2) 
Now, differentiating partially (1) w.r.t. ?? , we get 
??? ??? =?? +?? (?? 2
+?? 2
)·2?? 
So, 
?? -?? 2?? =?? '
(?? 2
+?? 2
) (3)
 
Equating (2) and (3), we get 
?? -?? 2?? =
?? -?? 2?? 
So, ???? -???? =?? 2
-?? 2
 is linear PDE. 
1.3 Find the surface satisfying the P.D.E. (?? ?? -?? ?? ?? '
+?? ?? )?? =?? and the 
conditions that ???? =?? ?? when ?? =?? and ???? =?? ?? when ?? =?? . 
(2010 : 12 Marks) 
Solution: 
Given, the equation is 
(?? 2
-2???? +?? 2
)?? =0 
?????????????????????????????????????????????????????????????????????????????(?? -?? '
)
2
?? =0 
The auxiliary eqn. for above eqn. is 
(?? -1)
2
=0 
??????????????????????????????????????????????????????????????????????????????????????? =1,1 
? The solution of above eqn. is 
?? =?? 1
(?? +?? )+?? ?? 2
(?? +?? )
 Given, at ?? =0,???? =?? 2
?
?? =
?? 2
?? i.e., ?
?? 2
?? =?? 1
(?? )+0??? 1
(?? )=
?? 2
?? ????? 1
(?? +?? )=
(?? +?? )
2
?? at ?? =0,???? =?? 2
???? =
?? 2
?? i.e., ?
?? 2
?? =?? 1
(?? )+?? ?? 2
(?? )
???
?? 2
?? =
?? 2
?? +?? ?? 2
(?? )
????? ?? 2
(?? )=?? 2
(
1
?? -
1
?? )??? 2
(?? )=?? (
1
?? -
1
?? )
 
???? 2
(?? +?? )=(?? +?? )(
1
?? -
1
?? ) 
? Putting these values of ?? 1
 and ?? 2
 in the solution, we get 
?? =
(?? +?? )
2
?? +?? (?? +?? )(
1
?? -
1
?? ) 
1.4 Find the surface satisfying 
?? ?? ?? ?? ?? ?? =?? ?? +?? and touching ?? =?? ?? +?? ?? along its 
section by the plane ?? +?? +?? =?? . 
(2011 : 20 Marks) 
Page 4


Edurev123 
Partial Differential Equations 
1. Formulation of P.D.E. 
1.1 Show that the differential equation of all cones which have their vertex at the 
origin is ???? +???? =?? . Verify that this equation is satisfied by the surface ???? +?? ?? +
???? =?? . 
(2009 : 12 Marks) 
Solution: 
The equation cone having vertex at origin 
?? ?? 2
+?? ?? 2
+?? ?? 2
+2h???? +2?????? +2?????? =0 (1) 
where ???,?? ,?? ,?? ,?? ,h are parameters. 
Differentiating w.r.t. ?? and ?? , we get 
2???? +2h?? +2???? +2?????? +2?????? +2?????? ?=0
2???? +2?????? +2h?? +2?????? +2???? +2?????? ?=0
???? +h?? +???? +?? (???? +???? +???? )?=0×?? ???? +h?? +???? +?? (???? +???? +???? )?=0×?? ?? ?? 2
+h???? +?????? +?? (?? ?? 2
+?????? +?????? )?=0
?? ?? 2
+h???? +?????? +?? (?????? +?? ?? 2
+?????? )?=0
 
On adding, 
???????????????? ?? 2
+?? ?? 2
+2h???? +?????? +?????? +???? +???? [???? +???? +???? ]=0?????????
???????????????????????????????-(?? ?? 2
+?????? +?????? )+(???? +???? +???? )(???? +???? )=0?????????
????????????????????????????????????????????????????????????????????(???? +???? +???? )(???? +???? -?? )=0?????????
 
Clearly, ???? +???? -?? =0 is required differential equation. 
Given surface is ???? +???? +???? =0 
Differentiating (?
*
) w.r.t. ?? and ?? , we get 
???? +?? +???? +?? =0 (2)
?? +???? +???? +?? =0 (3)
 
So, we get 
?? ?=
-(?? +?? )
(?? +?? )
,?? =
-(?? +?? )
(?? +?? )
???? +???? -?? ?=
-(?? +?? )?? (?? +?? )
-
(?? +?? )
(?? +?? )
?? -?? ?=
-(?? +?? )?? -(?? +?? )?? -?? (?? +?? )
(?? +?? )
?=
-???? -???? -???? -???? -???? -????
(?? +?? )
?=
-2(???? +???? +???? )
?? +?? =
-20
?? +?? =0
 
1.2 From the partial differential equation by eliminating the arbitrary function ?? 
given by: 
?? (?? ?? +?? ?? ,?? -???? )=?? 
Solution: 
The function is 
???????????????????????????????????????????????????????????????????? =???? +?? (?? 2
+?? 2
)??????????????????????????????????????????????????????????????(1) 
Now differentiating partially (1) w.r.t. ?? we get 
??? ??? =?? +?? '
(?? 2
+?? 2
)2?? 
So, 
?? -?? 2?? =?? '
(?? 2
+?? 2
) (2) 
Now, differentiating partially (1) w.r.t. ?? , we get 
??? ??? =?? +?? (?? 2
+?? 2
)·2?? 
So, 
?? -?? 2?? =?? '
(?? 2
+?? 2
) (3)
 
Equating (2) and (3), we get 
?? -?? 2?? =
?? -?? 2?? 
So, ???? -???? =?? 2
-?? 2
 is linear PDE. 
1.3 Find the surface satisfying the P.D.E. (?? ?? -?? ?? ?? '
+?? ?? )?? =?? and the 
conditions that ???? =?? ?? when ?? =?? and ???? =?? ?? when ?? =?? . 
(2010 : 12 Marks) 
Solution: 
Given, the equation is 
(?? 2
-2???? +?? 2
)?? =0 
?????????????????????????????????????????????????????????????????????????????(?? -?? '
)
2
?? =0 
The auxiliary eqn. for above eqn. is 
(?? -1)
2
=0 
??????????????????????????????????????????????????????????????????????????????????????? =1,1 
? The solution of above eqn. is 
?? =?? 1
(?? +?? )+?? ?? 2
(?? +?? )
 Given, at ?? =0,???? =?? 2
?
?? =
?? 2
?? i.e., ?
?? 2
?? =?? 1
(?? )+0??? 1
(?? )=
?? 2
?? ????? 1
(?? +?? )=
(?? +?? )
2
?? at ?? =0,???? =?? 2
???? =
?? 2
?? i.e., ?
?? 2
?? =?? 1
(?? )+?? ?? 2
(?? )
???
?? 2
?? =
?? 2
?? +?? ?? 2
(?? )
????? ?? 2
(?? )=?? 2
(
1
?? -
1
?? )??? 2
(?? )=?? (
1
?? -
1
?? )
 
???? 2
(?? +?? )=(?? +?? )(
1
?? -
1
?? ) 
? Putting these values of ?? 1
 and ?? 2
 in the solution, we get 
?? =
(?? +?? )
2
?? +?? (?? +?? )(
1
?? -
1
?? ) 
1.4 Find the surface satisfying 
?? ?? ?? ?? ?? ?? =?? ?? +?? and touching ?? =?? ?? +?? ?? along its 
section by the plane ?? +?? +?? =?? . 
(2011 : 20 Marks) 
Solution: 
Given : 
?
2
?? ??? 2
=6?? +2 
??
??? ??? =6?? +2 where ?? =
??? ??? (??)
 
Integrating (i) w.r.t. ?? , 
?? ?=3?? 2
+2?? +?? (?? ) 
??????????????????????????????????????????????????????????????
??? ??? =3?? 2
+2?? +?? (?? )????????????????????????????????????????????????????????????(???? ) 
Integrating (ii) w.r.t. ?? , 
?? =?? 3
+?? 2
+???? (?? )+?? (?? ) (?????? ) 
where ?? (?? ) and ?? (?? ) are arbitrary functions. 
The given surface is 
?? =?? 3
+?? 3
(???? ) 
and the given plane is 
?? +?? +1=0 (?? ) 
Since (iii) and (iv) touch each other, along their section by ( ?? ), the values of ?? and ?? at 
any point on ( ?? ) must be equal. Thus, we must have 
and ?????????????????????????????????????????????????????
3?? 2
+2?? +?? (?? )?=3?? 2
??????????????????????????????????????????????????????????????(???? )
???? (?? )+?? (?? )?=3?? 2
?????????????????????????????????????????????????????????????(?????? )
 
From (v) and (vi), 
?? (?? ) ?=-2?? =2(?? +1)???????????????????????????????????????????????(???????? )
?? (?? ) ?=2
from?(vii)????????????????????????????????????????2?? +?? (?? ) ?=3?? 2
?? (?? ) ?=3?? 2
-2?? ?=3?? 2
+2(?? +1)??????????????????????????????????????????????????(???? )
?? (?? ) ?=?? 3
+?? 2
+2?? +?? 
where ?? is an arbitrary constant. 
From (viii) and (ix), and using (iii), we get, 
Page 5


Edurev123 
Partial Differential Equations 
1. Formulation of P.D.E. 
1.1 Show that the differential equation of all cones which have their vertex at the 
origin is ???? +???? =?? . Verify that this equation is satisfied by the surface ???? +?? ?? +
???? =?? . 
(2009 : 12 Marks) 
Solution: 
The equation cone having vertex at origin 
?? ?? 2
+?? ?? 2
+?? ?? 2
+2h???? +2?????? +2?????? =0 (1) 
where ???,?? ,?? ,?? ,?? ,h are parameters. 
Differentiating w.r.t. ?? and ?? , we get 
2???? +2h?? +2???? +2?????? +2?????? +2?????? ?=0
2???? +2?????? +2h?? +2?????? +2???? +2?????? ?=0
???? +h?? +???? +?? (???? +???? +???? )?=0×?? ???? +h?? +???? +?? (???? +???? +???? )?=0×?? ?? ?? 2
+h???? +?????? +?? (?? ?? 2
+?????? +?????? )?=0
?? ?? 2
+h???? +?????? +?? (?????? +?? ?? 2
+?????? )?=0
 
On adding, 
???????????????? ?? 2
+?? ?? 2
+2h???? +?????? +?????? +???? +???? [???? +???? +???? ]=0?????????
???????????????????????????????-(?? ?? 2
+?????? +?????? )+(???? +???? +???? )(???? +???? )=0?????????
????????????????????????????????????????????????????????????????????(???? +???? +???? )(???? +???? -?? )=0?????????
 
Clearly, ???? +???? -?? =0 is required differential equation. 
Given surface is ???? +???? +???? =0 
Differentiating (?
*
) w.r.t. ?? and ?? , we get 
???? +?? +???? +?? =0 (2)
?? +???? +???? +?? =0 (3)
 
So, we get 
?? ?=
-(?? +?? )
(?? +?? )
,?? =
-(?? +?? )
(?? +?? )
???? +???? -?? ?=
-(?? +?? )?? (?? +?? )
-
(?? +?? )
(?? +?? )
?? -?? ?=
-(?? +?? )?? -(?? +?? )?? -?? (?? +?? )
(?? +?? )
?=
-???? -???? -???? -???? -???? -????
(?? +?? )
?=
-2(???? +???? +???? )
?? +?? =
-20
?? +?? =0
 
1.2 From the partial differential equation by eliminating the arbitrary function ?? 
given by: 
?? (?? ?? +?? ?? ,?? -???? )=?? 
Solution: 
The function is 
???????????????????????????????????????????????????????????????????? =???? +?? (?? 2
+?? 2
)??????????????????????????????????????????????????????????????(1) 
Now differentiating partially (1) w.r.t. ?? we get 
??? ??? =?? +?? '
(?? 2
+?? 2
)2?? 
So, 
?? -?? 2?? =?? '
(?? 2
+?? 2
) (2) 
Now, differentiating partially (1) w.r.t. ?? , we get 
??? ??? =?? +?? (?? 2
+?? 2
)·2?? 
So, 
?? -?? 2?? =?? '
(?? 2
+?? 2
) (3)
 
Equating (2) and (3), we get 
?? -?? 2?? =
?? -?? 2?? 
So, ???? -???? =?? 2
-?? 2
 is linear PDE. 
1.3 Find the surface satisfying the P.D.E. (?? ?? -?? ?? ?? '
+?? ?? )?? =?? and the 
conditions that ???? =?? ?? when ?? =?? and ???? =?? ?? when ?? =?? . 
(2010 : 12 Marks) 
Solution: 
Given, the equation is 
(?? 2
-2???? +?? 2
)?? =0 
?????????????????????????????????????????????????????????????????????????????(?? -?? '
)
2
?? =0 
The auxiliary eqn. for above eqn. is 
(?? -1)
2
=0 
??????????????????????????????????????????????????????????????????????????????????????? =1,1 
? The solution of above eqn. is 
?? =?? 1
(?? +?? )+?? ?? 2
(?? +?? )
 Given, at ?? =0,???? =?? 2
?
?? =
?? 2
?? i.e., ?
?? 2
?? =?? 1
(?? )+0??? 1
(?? )=
?? 2
?? ????? 1
(?? +?? )=
(?? +?? )
2
?? at ?? =0,???? =?? 2
???? =
?? 2
?? i.e., ?
?? 2
?? =?? 1
(?? )+?? ?? 2
(?? )
???
?? 2
?? =
?? 2
?? +?? ?? 2
(?? )
????? ?? 2
(?? )=?? 2
(
1
?? -
1
?? )??? 2
(?? )=?? (
1
?? -
1
?? )
 
???? 2
(?? +?? )=(?? +?? )(
1
?? -
1
?? ) 
? Putting these values of ?? 1
 and ?? 2
 in the solution, we get 
?? =
(?? +?? )
2
?? +?? (?? +?? )(
1
?? -
1
?? ) 
1.4 Find the surface satisfying 
?? ?? ?? ?? ?? ?? =?? ?? +?? and touching ?? =?? ?? +?? ?? along its 
section by the plane ?? +?? +?? =?? . 
(2011 : 20 Marks) 
Solution: 
Given : 
?
2
?? ??? 2
=6?? +2 
??
??? ??? =6?? +2 where ?? =
??? ??? (??)
 
Integrating (i) w.r.t. ?? , 
?? ?=3?? 2
+2?? +?? (?? ) 
??????????????????????????????????????????????????????????????
??? ??? =3?? 2
+2?? +?? (?? )????????????????????????????????????????????????????????????(???? ) 
Integrating (ii) w.r.t. ?? , 
?? =?? 3
+?? 2
+???? (?? )+?? (?? ) (?????? ) 
where ?? (?? ) and ?? (?? ) are arbitrary functions. 
The given surface is 
?? =?? 3
+?? 3
(???? ) 
and the given plane is 
?? +?? +1=0 (?? ) 
Since (iii) and (iv) touch each other, along their section by ( ?? ), the values of ?? and ?? at 
any point on ( ?? ) must be equal. Thus, we must have 
and ?????????????????????????????????????????????????????
3?? 2
+2?? +?? (?? )?=3?? 2
??????????????????????????????????????????????????????????????(???? )
???? (?? )+?? (?? )?=3?? 2
?????????????????????????????????????????????????????????????(?????? )
 
From (v) and (vi), 
?? (?? ) ?=-2?? =2(?? +1)???????????????????????????????????????????????(???????? )
?? (?? ) ?=2
from?(vii)????????????????????????????????????????2?? +?? (?? ) ?=3?? 2
?? (?? ) ?=3?? 2
-2?? ?=3?? 2
+2(?? +1)??????????????????????????????????????????????????(???? )
?? (?? ) ?=?? 3
+?? 2
+2?? +?? 
where ?? is an arbitrary constant. 
From (viii) and (ix), and using (iii), we get, 
?? =?? 3
+?? 2
+2?? (?? +1)+?? 3
+?? 2
+2?? +?? (?? ) 
At the point of contact of (iv) and ( x ) values of ?? must be the same and hence, we have 
?? 3
+?? 2
+2?? (?? +1)+?? 3
+?? 2
+2?? +?? =?? 3
+?? 3
                                                             
(xi) 
Using ?? =-?? -1 from (v), (xi) gives 
?? =1 
Putting ?? =1 in (?? ) , the required surface is 
?? ?=?? 3
+?? 3
+2?? (?? +1)+?? 3
+?? 2
+2?? +1
?=?? 3
+?? 3
+(?? +?? +1)
2
 
1.5 Form a partial differential equation by eliminating the arbitrary functions ?? and 
?? from ?? =???? (?? )+???? (?? ) . 
(2013: 10 marks) 
Solution: 
Differentiating partially with respect to ?? and ?? 
?? =???? (?? )+???? (?? ) 
 and 
??? ??? ?=?? ?? '
(?? )+?? (?? );
??? ??? =?? (?? )+?? ?? '
(?? )
?
2
?? ??? ??? ?=?? (?? )+?? '
(?? )
?? ??? ??? +?? ??? ??? ?=???? [?? (?? )+?? '
(?? )]+???? (?? )+???? (?? )
?=????
?
2
?? ??? ??? +?? ???? ??? ??? +?? ??? ??? -????
?
2
?? ??? ??? -?? ?=0
 
is the required partial differential equation. 
1.6 Find the surface which intersects the surface of the system 
?? (?? +?? )=?? (?? ?? +?? )(?? being a constant ) 
Orthogonally and which passes through the circle ?? ?? +?? ?? =?? ,?? =?? . 
(2013: 15 marks) 
Solution: 
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