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 Page 1


Edurev123 
2. Functions of Two and Three Variables 
2.1 If ?? =?? ±?? .???? and ?? =?? ±?? .???? with approximately what accuracy can you 
calculate the polar coordinates ?? and ?? of the point ?? (?? ,?? ) ? Express your 
estimates as percentage changes of the value that ?? and ?? have at point (?? ,?? ) . 
(2009 : 20 Marks) 
Solution: 
Approach : ?? and ?? are functions of two variables ?? and ?? . We write the total derivative of 
each to calculate error from the error in the variables. 
We have ?? =3,???? =0.01,?? =4,???? =0.01 
By definition 
?? =v?? 2
+?? 2
?? =tan
-1
 
?? ?? 
By total derivative 
???? =
??? ??? ???? +
??? ??? ????
 =
?? v?? 2
+?? 2
???? +
?? v?? 2
+?? 2
????
 =
3×0.01
5
+
4
5
×0.01=
7
5
×0.01
 
Percentage error in ?? 
 =
??? ?? ×100=
7
5
×0.01
v3
2
+4
2
×100
 =
7
25
=0.28%
 
Similarly,                                 ?? =tan
-1
 
?? ?? =tan
-1
 
4
3
 
Page 2


Edurev123 
2. Functions of Two and Three Variables 
2.1 If ?? =?? ±?? .???? and ?? =?? ±?? .???? with approximately what accuracy can you 
calculate the polar coordinates ?? and ?? of the point ?? (?? ,?? ) ? Express your 
estimates as percentage changes of the value that ?? and ?? have at point (?? ,?? ) . 
(2009 : 20 Marks) 
Solution: 
Approach : ?? and ?? are functions of two variables ?? and ?? . We write the total derivative of 
each to calculate error from the error in the variables. 
We have ?? =3,???? =0.01,?? =4,???? =0.01 
By definition 
?? =v?? 2
+?? 2
?? =tan
-1
 
?? ?? 
By total derivative 
???? =
??? ??? ???? +
??? ??? ????
 =
?? v?? 2
+?? 2
???? +
?? v?? 2
+?? 2
????
 =
3×0.01
5
+
4
5
×0.01=
7
5
×0.01
 
Percentage error in ?? 
 =
??? ?? ×100=
7
5
×0.01
v3
2
+4
2
×100
 =
7
25
=0.28%
 
Similarly,                                 ?? =tan
-1
 
?? ?? =tan
-1
 
4
3
 
            ???? =
??? ??? ???? +
??? ??? ????
 =
1
1+
?? 2
?? 2
-
?? ?? 2
???? +
1
1+
?? 2
?? 2
-
1
?? ????
 =
-?? ?? 2
+?? 2
???? +
?? ?? 2
+?? 2
????
?                                                    ???? =
-4×0.01
25
+
3
25
×0.01=
-1
25
×0.01
?                       Percentage error  =
1
2.5
tan
-1
 
4
3
×0.01×100=0.043%
 
 
2.2 Let ?? :?? ?? ??? be defined as 
?? (?? ,?? )={
????
v?? ?? +?? ?? if (?? ,?? )?(?? ,?? )
?? if (?? ,?? )=(?? ,?? )
 
Is ?? continuous at (?? ,?? ) ? Compute partial derivatives of ?? at any point (?? ,?? ) if 
exist. 
(2009 : 20 Marks) 
Solution: 
Approach : Using definition of continuity and partial derivative. 
Function being rational is continuous at all (?? ,?? ) except possibly at (0,0) . 
Continuity at (0,0) : 
To check whether lim
(?? ,?? )?(0,0)
?
????
v?? 2
+?? 2
=0. 
Converting to polar coordinates with ?? =?? cos ?? ,?? =?? sin ?? ,lim
(?? ,?? )?(0,0)
?~lim
?? ?0
? 
lim
?? ?0
?
?? 2
sin ?? cos ?? v?? 2
cos
2
 ?? +?? 2
sin
2
 ?? =lim
?? ?0
?
?? 2
sin ?? cos ?? ?? (?? is always +???? )
 =lim
?? ?0
??? sin ?? cos ?? =0 for all values of ?? 
??? (?? ,?? ) is cont. at (0,0) as well. 
Partial derivative at (?? ,?? )?(0,0) 
Page 3


Edurev123 
2. Functions of Two and Three Variables 
2.1 If ?? =?? ±?? .???? and ?? =?? ±?? .???? with approximately what accuracy can you 
calculate the polar coordinates ?? and ?? of the point ?? (?? ,?? ) ? Express your 
estimates as percentage changes of the value that ?? and ?? have at point (?? ,?? ) . 
(2009 : 20 Marks) 
Solution: 
Approach : ?? and ?? are functions of two variables ?? and ?? . We write the total derivative of 
each to calculate error from the error in the variables. 
We have ?? =3,???? =0.01,?? =4,???? =0.01 
By definition 
?? =v?? 2
+?? 2
?? =tan
-1
 
?? ?? 
By total derivative 
???? =
??? ??? ???? +
??? ??? ????
 =
?? v?? 2
+?? 2
???? +
?? v?? 2
+?? 2
????
 =
3×0.01
5
+
4
5
×0.01=
7
5
×0.01
 
Percentage error in ?? 
 =
??? ?? ×100=
7
5
×0.01
v3
2
+4
2
×100
 =
7
25
=0.28%
 
Similarly,                                 ?? =tan
-1
 
?? ?? =tan
-1
 
4
3
 
            ???? =
??? ??? ???? +
??? ??? ????
 =
1
1+
?? 2
?? 2
-
?? ?? 2
???? +
1
1+
?? 2
?? 2
-
1
?? ????
 =
-?? ?? 2
+?? 2
???? +
?? ?? 2
+?? 2
????
?                                                    ???? =
-4×0.01
25
+
3
25
×0.01=
-1
25
×0.01
?                       Percentage error  =
1
2.5
tan
-1
 
4
3
×0.01×100=0.043%
 
 
2.2 Let ?? :?? ?? ??? be defined as 
?? (?? ,?? )={
????
v?? ?? +?? ?? if (?? ,?? )?(?? ,?? )
?? if (?? ,?? )=(?? ,?? )
 
Is ?? continuous at (?? ,?? ) ? Compute partial derivatives of ?? at any point (?? ,?? ) if 
exist. 
(2009 : 20 Marks) 
Solution: 
Approach : Using definition of continuity and partial derivative. 
Function being rational is continuous at all (?? ,?? ) except possibly at (0,0) . 
Continuity at (0,0) : 
To check whether lim
(?? ,?? )?(0,0)
?
????
v?? 2
+?? 2
=0. 
Converting to polar coordinates with ?? =?? cos ?? ,?? =?? sin ?? ,lim
(?? ,?? )?(0,0)
?~lim
?? ?0
? 
lim
?? ?0
?
?? 2
sin ?? cos ?? v?? 2
cos
2
 ?? +?? 2
sin
2
 ?? =lim
?? ?0
?
?? 2
sin ?? cos ?? ?? (?? is always +???? )
 =lim
?? ?0
??? sin ?? cos ?? =0 for all values of ?? 
??? (?? ,?? ) is cont. at (0,0) as well. 
Partial derivative at (?? ,?? )?(0,0) 
??? ??? =
?? v?? 2
+?? 2
-???? ·
2?? 2
v?? 2
+?? 2
(?? 2
+?? 2
)
=
?? (?? 2
+?? 2
)-?? ?? 2
(?? 2
+?? 2
)
3/2
 =
?? 3
(?? 2
+?? 2
)
3/2
??? ??? =
?? v?? 2
+?? 2
-???? ·
2?? 2v?? 2
+?? 2
(?? 2
+?? 2
)
=
?? 3
(?? 2
+?? 2
)
3/2
 
                                                               (Simply ?? ??? as it is symmetrical function in ?? 
and ?? ) 
Partial derivative at (0,0) 
??? ??? =lim
h?0
?
?? (h,0)-?? (0,0)
h
=lim
h?0
?
0-0
h
=0
??? ??? =lim
?? ?0
?
?? (0,?? )-?? (0,0)
?? =lim
?? ?0
?
0-0
?? =0
 
2.3 A space probe in the shape of the ellipsoid ?? ?? ?? +?? ?? +?? ?? ?? =???? enters the 
earth's atmosphere and its surface begins to heat. After one hour the temperature 
at the point (?? ,?? ,?? ) on the probe surface is given by 
?? (?? ,?? ,?? )=?? ?? ?? +?? ???? -???? ?? +?????? 
Find hottest point on the probe surface. 
(2009 : 20 Marks) 
Solution: 
Approach: Use Lagrange's multipliers. 
                                          Max.?? (?? ,?? ,?? )=8?? 2
+4???? -16?? +600                             …(i)                
?????????????? ???? 4?? 2
+?? 2
+4?? 2
=1                                                     …(???? ) 
??????                          ?? =(8?? 2
+4???? -16?? +600)+?? (4?? 2
+?? 2
+4?? 2
-16) 
Taking total derivative of ??  
???? =(16+8?? )?????? +(4?? +2???? )???? +(8???? -16)???? +4?? 
For maximum/minimum 
                              ???? =0?(16+8?? )?? =0;4?? +2???? =0;8???? +4?? -16=0               …(iii)    
Let ?? ?0? 
?? =-
1
2
 
Page 4


Edurev123 
2. Functions of Two and Three Variables 
2.1 If ?? =?? ±?? .???? and ?? =?? ±?? .???? with approximately what accuracy can you 
calculate the polar coordinates ?? and ?? of the point ?? (?? ,?? ) ? Express your 
estimates as percentage changes of the value that ?? and ?? have at point (?? ,?? ) . 
(2009 : 20 Marks) 
Solution: 
Approach : ?? and ?? are functions of two variables ?? and ?? . We write the total derivative of 
each to calculate error from the error in the variables. 
We have ?? =3,???? =0.01,?? =4,???? =0.01 
By definition 
?? =v?? 2
+?? 2
?? =tan
-1
 
?? ?? 
By total derivative 
???? =
??? ??? ???? +
??? ??? ????
 =
?? v?? 2
+?? 2
???? +
?? v?? 2
+?? 2
????
 =
3×0.01
5
+
4
5
×0.01=
7
5
×0.01
 
Percentage error in ?? 
 =
??? ?? ×100=
7
5
×0.01
v3
2
+4
2
×100
 =
7
25
=0.28%
 
Similarly,                                 ?? =tan
-1
 
?? ?? =tan
-1
 
4
3
 
            ???? =
??? ??? ???? +
??? ??? ????
 =
1
1+
?? 2
?? 2
-
?? ?? 2
???? +
1
1+
?? 2
?? 2
-
1
?? ????
 =
-?? ?? 2
+?? 2
???? +
?? ?? 2
+?? 2
????
?                                                    ???? =
-4×0.01
25
+
3
25
×0.01=
-1
25
×0.01
?                       Percentage error  =
1
2.5
tan
-1
 
4
3
×0.01×100=0.043%
 
 
2.2 Let ?? :?? ?? ??? be defined as 
?? (?? ,?? )={
????
v?? ?? +?? ?? if (?? ,?? )?(?? ,?? )
?? if (?? ,?? )=(?? ,?? )
 
Is ?? continuous at (?? ,?? ) ? Compute partial derivatives of ?? at any point (?? ,?? ) if 
exist. 
(2009 : 20 Marks) 
Solution: 
Approach : Using definition of continuity and partial derivative. 
Function being rational is continuous at all (?? ,?? ) except possibly at (0,0) . 
Continuity at (0,0) : 
To check whether lim
(?? ,?? )?(0,0)
?
????
v?? 2
+?? 2
=0. 
Converting to polar coordinates with ?? =?? cos ?? ,?? =?? sin ?? ,lim
(?? ,?? )?(0,0)
?~lim
?? ?0
? 
lim
?? ?0
?
?? 2
sin ?? cos ?? v?? 2
cos
2
 ?? +?? 2
sin
2
 ?? =lim
?? ?0
?
?? 2
sin ?? cos ?? ?? (?? is always +???? )
 =lim
?? ?0
??? sin ?? cos ?? =0 for all values of ?? 
??? (?? ,?? ) is cont. at (0,0) as well. 
Partial derivative at (?? ,?? )?(0,0) 
??? ??? =
?? v?? 2
+?? 2
-???? ·
2?? 2
v?? 2
+?? 2
(?? 2
+?? 2
)
=
?? (?? 2
+?? 2
)-?? ?? 2
(?? 2
+?? 2
)
3/2
 =
?? 3
(?? 2
+?? 2
)
3/2
??? ??? =
?? v?? 2
+?? 2
-???? ·
2?? 2v?? 2
+?? 2
(?? 2
+?? 2
)
=
?? 3
(?? 2
+?? 2
)
3/2
 
                                                               (Simply ?? ??? as it is symmetrical function in ?? 
and ?? ) 
Partial derivative at (0,0) 
??? ??? =lim
h?0
?
?? (h,0)-?? (0,0)
h
=lim
h?0
?
0-0
h
=0
??? ??? =lim
?? ?0
?
?? (0,?? )-?? (0,0)
?? =lim
?? ?0
?
0-0
?? =0
 
2.3 A space probe in the shape of the ellipsoid ?? ?? ?? +?? ?? +?? ?? ?? =???? enters the 
earth's atmosphere and its surface begins to heat. After one hour the temperature 
at the point (?? ,?? ,?? ) on the probe surface is given by 
?? (?? ,?? ,?? )=?? ?? ?? +?? ???? -???? ?? +?????? 
Find hottest point on the probe surface. 
(2009 : 20 Marks) 
Solution: 
Approach: Use Lagrange's multipliers. 
                                          Max.?? (?? ,?? ,?? )=8?? 2
+4???? -16?? +600                             …(i)                
?????????????? ???? 4?? 2
+?? 2
+4?? 2
=1                                                     …(???? ) 
??????                          ?? =(8?? 2
+4???? -16?? +600)+?? (4?? 2
+?? 2
+4?? 2
-16) 
Taking total derivative of ??  
???? =(16+8?? )?????? +(4?? +2???? )???? +(8???? -16)???? +4?? 
For maximum/minimum 
                              ???? =0?(16+8?? )?? =0;4?? +2???? =0;8???? +4?? -16=0               …(iii)    
Let ?? ?0? 
?? =-
1
2
 
4?? -?? =0
-4z+4?? =16
}?? =
16
3
;?? =
4
3
 
Putting this in (iii) 
?? 2
=4-
1
4
(
256
9
+
64
9
)=
-44
9
<0 
So this value is not possible. 
?                                                                  x=0 
4?? +2???? =0 
8???? +4?? =16 
                                                (4?? 2
-4)?? =-16 
?                                                                  y=
4
1-?? 2
 
And                                                         z=
-?? 
?? 2
=
2?? ?? 2
-1
 
Substituting in (ii) 
                                       
16
(1-?? 2
)
2
+
16?? 2
(1-?? 2
)
2
=16
 ?                                                               1+?? 2
=(1-?? 2
)
2
 ?                                                              1+?? 2
=?? 4
-2?? 2
+1
 ?                                                      ?? 2
(?? 2
-3)=0
 ?                                                                        ?? =0;?? 2
=3
                                                                              ?? =0??? =0,?? =4,?? =0
 
Let us consider ?? and ?? as independent and ?? as dependent variable. 
(Note: We can do this because we have been given a relation between ?? ,?? and ?? ) 
From (ii) partially differentiating w.r.t. ?? . 
8?? +8?? ??? ??? =0?
??? ??? =-
?? ?? ?                                                         
??? ??? =16?? +4?? ??? ??? -16
??? ??? =16?? -
4????
?? +
16?? ?? ?
2
?? ??? 2
 =16+(16-4?? )(
1
?? -
?? ?? 2
·
??? ??? )
 =16+(16-4?? )(
1
?? +
?? 2
?? 3
)
 
Page 5


Edurev123 
2. Functions of Two and Three Variables 
2.1 If ?? =?? ±?? .???? and ?? =?? ±?? .???? with approximately what accuracy can you 
calculate the polar coordinates ?? and ?? of the point ?? (?? ,?? ) ? Express your 
estimates as percentage changes of the value that ?? and ?? have at point (?? ,?? ) . 
(2009 : 20 Marks) 
Solution: 
Approach : ?? and ?? are functions of two variables ?? and ?? . We write the total derivative of 
each to calculate error from the error in the variables. 
We have ?? =3,???? =0.01,?? =4,???? =0.01 
By definition 
?? =v?? 2
+?? 2
?? =tan
-1
 
?? ?? 
By total derivative 
???? =
??? ??? ???? +
??? ??? ????
 =
?? v?? 2
+?? 2
???? +
?? v?? 2
+?? 2
????
 =
3×0.01
5
+
4
5
×0.01=
7
5
×0.01
 
Percentage error in ?? 
 =
??? ?? ×100=
7
5
×0.01
v3
2
+4
2
×100
 =
7
25
=0.28%
 
Similarly,                                 ?? =tan
-1
 
?? ?? =tan
-1
 
4
3
 
            ???? =
??? ??? ???? +
??? ??? ????
 =
1
1+
?? 2
?? 2
-
?? ?? 2
???? +
1
1+
?? 2
?? 2
-
1
?? ????
 =
-?? ?? 2
+?? 2
???? +
?? ?? 2
+?? 2
????
?                                                    ???? =
-4×0.01
25
+
3
25
×0.01=
-1
25
×0.01
?                       Percentage error  =
1
2.5
tan
-1
 
4
3
×0.01×100=0.043%
 
 
2.2 Let ?? :?? ?? ??? be defined as 
?? (?? ,?? )={
????
v?? ?? +?? ?? if (?? ,?? )?(?? ,?? )
?? if (?? ,?? )=(?? ,?? )
 
Is ?? continuous at (?? ,?? ) ? Compute partial derivatives of ?? at any point (?? ,?? ) if 
exist. 
(2009 : 20 Marks) 
Solution: 
Approach : Using definition of continuity and partial derivative. 
Function being rational is continuous at all (?? ,?? ) except possibly at (0,0) . 
Continuity at (0,0) : 
To check whether lim
(?? ,?? )?(0,0)
?
????
v?? 2
+?? 2
=0. 
Converting to polar coordinates with ?? =?? cos ?? ,?? =?? sin ?? ,lim
(?? ,?? )?(0,0)
?~lim
?? ?0
? 
lim
?? ?0
?
?? 2
sin ?? cos ?? v?? 2
cos
2
 ?? +?? 2
sin
2
 ?? =lim
?? ?0
?
?? 2
sin ?? cos ?? ?? (?? is always +???? )
 =lim
?? ?0
??? sin ?? cos ?? =0 for all values of ?? 
??? (?? ,?? ) is cont. at (0,0) as well. 
Partial derivative at (?? ,?? )?(0,0) 
??? ??? =
?? v?? 2
+?? 2
-???? ·
2?? 2
v?? 2
+?? 2
(?? 2
+?? 2
)
=
?? (?? 2
+?? 2
)-?? ?? 2
(?? 2
+?? 2
)
3/2
 =
?? 3
(?? 2
+?? 2
)
3/2
??? ??? =
?? v?? 2
+?? 2
-???? ·
2?? 2v?? 2
+?? 2
(?? 2
+?? 2
)
=
?? 3
(?? 2
+?? 2
)
3/2
 
                                                               (Simply ?? ??? as it is symmetrical function in ?? 
and ?? ) 
Partial derivative at (0,0) 
??? ??? =lim
h?0
?
?? (h,0)-?? (0,0)
h
=lim
h?0
?
0-0
h
=0
??? ??? =lim
?? ?0
?
?? (0,?? )-?? (0,0)
?? =lim
?? ?0
?
0-0
?? =0
 
2.3 A space probe in the shape of the ellipsoid ?? ?? ?? +?? ?? +?? ?? ?? =???? enters the 
earth's atmosphere and its surface begins to heat. After one hour the temperature 
at the point (?? ,?? ,?? ) on the probe surface is given by 
?? (?? ,?? ,?? )=?? ?? ?? +?? ???? -???? ?? +?????? 
Find hottest point on the probe surface. 
(2009 : 20 Marks) 
Solution: 
Approach: Use Lagrange's multipliers. 
                                          Max.?? (?? ,?? ,?? )=8?? 2
+4???? -16?? +600                             …(i)                
?????????????? ???? 4?? 2
+?? 2
+4?? 2
=1                                                     …(???? ) 
??????                          ?? =(8?? 2
+4???? -16?? +600)+?? (4?? 2
+?? 2
+4?? 2
-16) 
Taking total derivative of ??  
???? =(16+8?? )?????? +(4?? +2???? )???? +(8???? -16)???? +4?? 
For maximum/minimum 
                              ???? =0?(16+8?? )?? =0;4?? +2???? =0;8???? +4?? -16=0               …(iii)    
Let ?? ?0? 
?? =-
1
2
 
4?? -?? =0
-4z+4?? =16
}?? =
16
3
;?? =
4
3
 
Putting this in (iii) 
?? 2
=4-
1
4
(
256
9
+
64
9
)=
-44
9
<0 
So this value is not possible. 
?                                                                  x=0 
4?? +2???? =0 
8???? +4?? =16 
                                                (4?? 2
-4)?? =-16 
?                                                                  y=
4
1-?? 2
 
And                                                         z=
-?? 
?? 2
=
2?? ?? 2
-1
 
Substituting in (ii) 
                                       
16
(1-?? 2
)
2
+
16?? 2
(1-?? 2
)
2
=16
 ?                                                               1+?? 2
=(1-?? 2
)
2
 ?                                                              1+?? 2
=?? 4
-2?? 2
+1
 ?                                                      ?? 2
(?? 2
-3)=0
 ?                                                                        ?? =0;?? 2
=3
                                                                              ?? =0??? =0,?? =4,?? =0
 
Let us consider ?? and ?? as independent and ?? as dependent variable. 
(Note: We can do this because we have been given a relation between ?? ,?? and ?? ) 
From (ii) partially differentiating w.r.t. ?? . 
8?? +8?? ??? ??? =0?
??? ??? =-
?? ?? ?                                                         
??? ??? =16?? +4?? ??? ??? -16
??? ??? =16?? -
4????
?? +
16?? ?? ?
2
?? ??? 2
 =16+(16-4?? )(
1
?? -
?? ?? 2
·
??? ??? )
 =16+(16-4?? )(
1
?? +
?? 2
?? 3
)
 
At ?? =4,
?
2
?? ??? 2
>0, so this cannot be maxima. 
?? 2
= 3,?? =±v3
?? =v3,?? =v3,?? =-2,?? =0
 
Again 
?
2
?? ??? 2
>0 so this can not be a maxima. 
?? =-v3,?? =-v3,?? =-2,?? =0
?
2
?? ??? 2
 =16-
24
v3
<0
 
3o, this is a maxima. 
? Hottest point is (0,-2,-v3) . 
2.4 Evaluate :?? =?
?? ??????????? +???????? +?? ?? ?? ???????? where ?? is the outer surface of the 
part of the sphere ?? ?? + ?? ?? +?? ?? =?? in the first quadrant. 
(2009 : 20 Marks) 
Solution: 
Approach: Surface integral with two changing variables can be calculated by taking the 
projection of the surface in the required plane. Use of polar coordinates can simplify 
integration. 
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