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Edurev123 
6. Paraboloid and it Properties 
6.1 Show that the plane ?? ?? +?? ?? +?? ?? +
?? ?? =?? touches the paraboloid ?? ?? ?? +?? ?? ?? =
???? ?? and find the point of contact. 
(2010 : 20 Marks) 
Solution: 
Given plane 
?? =3?? +3?? +7?? +
5
2
=0 (??)
 
and 
 Paraboloid =3?? 2
+4?? 2
=10?? 
Now, let point of contact be (?? 1
,?? 1
,?? 1
) . 
? If given plane touches paraboloid at this point of contact, then the plane should be 
tangent plane to the paraboloid. 
Now, equation of tangent plane at (?? 1
,?? 1
,?? 1
) is 
3?? ?? 1
+4?? ?? 1
=
10(?? +?? 1
)
2
 
?                                    3?? ?? 1
+4?? ?? 1
-5?? -5?? 1
=0 
Comparing (1) and (2), we get 
      
3
3?? 1
=
4
4?? 1
=
7
-5
=
5/2
-521
 
?                                                               
3
3?? 1
=
-7
5
??? 1
=
-5
7
 
4
4?? 1
=
7
-5
??? 1
=
-5
7
 
?                                                                
7
-5
=
5
-5×221
??? 1
=
5
14
 
?              Point of contact (?? 1
,?? 1
,?? 1
) =(
-5
7
,
-5
7
,
5
14
) 
Page 2


Edurev123 
6. Paraboloid and it Properties 
6.1 Show that the plane ?? ?? +?? ?? +?? ?? +
?? ?? =?? touches the paraboloid ?? ?? ?? +?? ?? ?? =
???? ?? and find the point of contact. 
(2010 : 20 Marks) 
Solution: 
Given plane 
?? =3?? +3?? +7?? +
5
2
=0 (??)
 
and 
 Paraboloid =3?? 2
+4?? 2
=10?? 
Now, let point of contact be (?? 1
,?? 1
,?? 1
) . 
? If given plane touches paraboloid at this point of contact, then the plane should be 
tangent plane to the paraboloid. 
Now, equation of tangent plane at (?? 1
,?? 1
,?? 1
) is 
3?? ?? 1
+4?? ?? 1
=
10(?? +?? 1
)
2
 
?                                    3?? ?? 1
+4?? ?? 1
-5?? -5?? 1
=0 
Comparing (1) and (2), we get 
      
3
3?? 1
=
4
4?? 1
=
7
-5
=
5/2
-521
 
?                                                               
3
3?? 1
=
-7
5
??? 1
=
-5
7
 
4
4?? 1
=
7
-5
??? 1
=
-5
7
 
?                                                                
7
-5
=
5
-5×221
??? 1
=
5
14
 
?              Point of contact (?? 1
,?? 1
,?? 1
) =(
-5
7
,
-5
7
,
5
14
) 
6.2. Show that the locus of a point from which the three mutually perpendicular 
tangent lines can be drawn to the paraboloid ?? ?? +?? ?? +?? ?? =?? is 
?? ?? +?? ?? +?? ?? =?? 
(2012 : 20 Marks) 
Solution: 
Let ?? (?? 1
,?? 1
,?? 1
) be the point from which three mutually perpendicular lines can be drawn 
to the paraboloid 
?? 2
+?? 2
+2?? =0 (??) 
Then, the enveloping cone of (i) with the vertex at ?? 
(?? 1
,?? 1
,?? 1
) is 
?? ?? 1
 =?? 2
(???? )
?? =?? 2
+?? 2
+2?? ?? 1
 =?? 1
2
+?? 1
2
+2?? 1
?? =?? ?? 1
+?? ?? 1
+?? +?? 1
 
? from (ii), we have 
(?? 2
+?? 2
+2?? )(?? 1
2
+?? 1
2
+2?? 1
)=(?? ?? 1
+?? ?? 1
+(?? +?? 1
))
2
 
For three mutually perpendicular generators, coefficient of ?? 2
+ coefficient of ?? 2
+ 
coefficient of ?? 2
=0 
or 
?? 1
2
+?? 1
2
+4?? 1
-1 =0
?? 1
2
+?? 1
2
+4?? 1
 =0
 
? Locus of (?? 1
,?? 1
,?? 1
) is  
         ?? 2
+?? 2
+4?? =1 
6.3 Two perpendicular tangent planes to the parabolold ?? ?? +?? ?? =?? ?? intersect in a 
straight line in the plane ?? =?? . Obtain the curve to which this straight line 
touches. 
(2015 : 13 Marks) 
Solution: 
Let the line of intersection of the two planes be: 
???? +???? =?? ,?? =0 (??) 
Page 3


Edurev123 
6. Paraboloid and it Properties 
6.1 Show that the plane ?? ?? +?? ?? +?? ?? +
?? ?? =?? touches the paraboloid ?? ?? ?? +?? ?? ?? =
???? ?? and find the point of contact. 
(2010 : 20 Marks) 
Solution: 
Given plane 
?? =3?? +3?? +7?? +
5
2
=0 (??)
 
and 
 Paraboloid =3?? 2
+4?? 2
=10?? 
Now, let point of contact be (?? 1
,?? 1
,?? 1
) . 
? If given plane touches paraboloid at this point of contact, then the plane should be 
tangent plane to the paraboloid. 
Now, equation of tangent plane at (?? 1
,?? 1
,?? 1
) is 
3?? ?? 1
+4?? ?? 1
=
10(?? +?? 1
)
2
 
?                                    3?? ?? 1
+4?? ?? 1
-5?? -5?? 1
=0 
Comparing (1) and (2), we get 
      
3
3?? 1
=
4
4?? 1
=
7
-5
=
5/2
-521
 
?                                                               
3
3?? 1
=
-7
5
??? 1
=
-5
7
 
4
4?? 1
=
7
-5
??? 1
=
-5
7
 
?                                                                
7
-5
=
5
-5×221
??? 1
=
5
14
 
?              Point of contact (?? 1
,?? 1
,?? 1
) =(
-5
7
,
-5
7
,
5
14
) 
6.2. Show that the locus of a point from which the three mutually perpendicular 
tangent lines can be drawn to the paraboloid ?? ?? +?? ?? +?? ?? =?? is 
?? ?? +?? ?? +?? ?? =?? 
(2012 : 20 Marks) 
Solution: 
Let ?? (?? 1
,?? 1
,?? 1
) be the point from which three mutually perpendicular lines can be drawn 
to the paraboloid 
?? 2
+?? 2
+2?? =0 (??) 
Then, the enveloping cone of (i) with the vertex at ?? 
(?? 1
,?? 1
,?? 1
) is 
?? ?? 1
 =?? 2
(???? )
?? =?? 2
+?? 2
+2?? ?? 1
 =?? 1
2
+?? 1
2
+2?? 1
?? =?? ?? 1
+?? ?? 1
+?? +?? 1
 
? from (ii), we have 
(?? 2
+?? 2
+2?? )(?? 1
2
+?? 1
2
+2?? 1
)=(?? ?? 1
+?? ?? 1
+(?? +?? 1
))
2
 
For three mutually perpendicular generators, coefficient of ?? 2
+ coefficient of ?? 2
+ 
coefficient of ?? 2
=0 
or 
?? 1
2
+?? 1
2
+4?? 1
-1 =0
?? 1
2
+?? 1
2
+4?? 1
 =0
 
? Locus of (?? 1
,?? 1
,?? 1
) is  
         ?? 2
+?? 2
+4?? =1 
6.3 Two perpendicular tangent planes to the parabolold ?? ?? +?? ?? =?? ?? intersect in a 
straight line in the plane ?? =?? . Obtain the curve to which this straight line 
touches. 
(2015 : 13 Marks) 
Solution: 
Let the line of intersection of the two planes be: 
???? +???? =?? ,?? =0 (??) 
Since this lies on the plane ?? =0 (given). 
? Equation of the plane through the line (i) is 
(???? +???? -?? )+???? =0
???? +???? +???? =?? (???? ) 
If the plane (ii) touches the paraboloid, then 
?? 2
?? 2
+
?? 2
?? 2
+
2????
?? =0 (condition)  
i.e., 
?? 2
+?? 2
+2???? =0 (?????? ) 
This being quadratic in ?? , gives two values of ?? 1
 say ?? 1
 and ?? 2
 such that 
?? 1
·?? 2
=
?? 2
+2????
?? (???? )
 
Also from (ii), the direction ratio's of the normal to the two tangent planes whose line of 
intersection is (ii) are ?? 1
,?? ,?? and ?? 2
,?? ,?? . 
Also, as these two tangent planes are perpendicular 
?                                                              ?? 1
·?? 2
+?? ·?? +?? ·?? =0 
?                                                            (?? 3
+2???? )+?? 2
+?? 2
=0                                          from (iv) 
?                                                                        2?? 2
+?? 2
+2???? =0                                                     (v) 
Now, we are to prove that the line (i) touches a parabola (to be found). So, we are to find 
the envelope of (i) which satisfies the condition (v). 
Eliminating ?? between (i) and (?? ) , the equations of the line of intersection of two tangent 
planes is : 
2?? 2
+?? 2
+2(???? +???? )?? =0,?? =0 
?                                           2(
?? ?? )
2
+2?? (
?? ?? )+(1+2?? )=0,?? =0 
It is quadratic in 
?? ?? , so its envelope is given by: 
?? 2
-4???? =0,?? =0 
?                                          (2?? )
2
-4·2(1+2?? )=0,?? =0 
?                                                                            ?? 2
=2(2?? +1),?? =0 
Page 4


Edurev123 
6. Paraboloid and it Properties 
6.1 Show that the plane ?? ?? +?? ?? +?? ?? +
?? ?? =?? touches the paraboloid ?? ?? ?? +?? ?? ?? =
???? ?? and find the point of contact. 
(2010 : 20 Marks) 
Solution: 
Given plane 
?? =3?? +3?? +7?? +
5
2
=0 (??)
 
and 
 Paraboloid =3?? 2
+4?? 2
=10?? 
Now, let point of contact be (?? 1
,?? 1
,?? 1
) . 
? If given plane touches paraboloid at this point of contact, then the plane should be 
tangent plane to the paraboloid. 
Now, equation of tangent plane at (?? 1
,?? 1
,?? 1
) is 
3?? ?? 1
+4?? ?? 1
=
10(?? +?? 1
)
2
 
?                                    3?? ?? 1
+4?? ?? 1
-5?? -5?? 1
=0 
Comparing (1) and (2), we get 
      
3
3?? 1
=
4
4?? 1
=
7
-5
=
5/2
-521
 
?                                                               
3
3?? 1
=
-7
5
??? 1
=
-5
7
 
4
4?? 1
=
7
-5
??? 1
=
-5
7
 
?                                                                
7
-5
=
5
-5×221
??? 1
=
5
14
 
?              Point of contact (?? 1
,?? 1
,?? 1
) =(
-5
7
,
-5
7
,
5
14
) 
6.2. Show that the locus of a point from which the three mutually perpendicular 
tangent lines can be drawn to the paraboloid ?? ?? +?? ?? +?? ?? =?? is 
?? ?? +?? ?? +?? ?? =?? 
(2012 : 20 Marks) 
Solution: 
Let ?? (?? 1
,?? 1
,?? 1
) be the point from which three mutually perpendicular lines can be drawn 
to the paraboloid 
?? 2
+?? 2
+2?? =0 (??) 
Then, the enveloping cone of (i) with the vertex at ?? 
(?? 1
,?? 1
,?? 1
) is 
?? ?? 1
 =?? 2
(???? )
?? =?? 2
+?? 2
+2?? ?? 1
 =?? 1
2
+?? 1
2
+2?? 1
?? =?? ?? 1
+?? ?? 1
+?? +?? 1
 
? from (ii), we have 
(?? 2
+?? 2
+2?? )(?? 1
2
+?? 1
2
+2?? 1
)=(?? ?? 1
+?? ?? 1
+(?? +?? 1
))
2
 
For three mutually perpendicular generators, coefficient of ?? 2
+ coefficient of ?? 2
+ 
coefficient of ?? 2
=0 
or 
?? 1
2
+?? 1
2
+4?? 1
-1 =0
?? 1
2
+?? 1
2
+4?? 1
 =0
 
? Locus of (?? 1
,?? 1
,?? 1
) is  
         ?? 2
+?? 2
+4?? =1 
6.3 Two perpendicular tangent planes to the parabolold ?? ?? +?? ?? =?? ?? intersect in a 
straight line in the plane ?? =?? . Obtain the curve to which this straight line 
touches. 
(2015 : 13 Marks) 
Solution: 
Let the line of intersection of the two planes be: 
???? +???? =?? ,?? =0 (??) 
Since this lies on the plane ?? =0 (given). 
? Equation of the plane through the line (i) is 
(???? +???? -?? )+???? =0
???? +???? +???? =?? (???? ) 
If the plane (ii) touches the paraboloid, then 
?? 2
?? 2
+
?? 2
?? 2
+
2????
?? =0 (condition)  
i.e., 
?? 2
+?? 2
+2???? =0 (?????? ) 
This being quadratic in ?? , gives two values of ?? 1
 say ?? 1
 and ?? 2
 such that 
?? 1
·?? 2
=
?? 2
+2????
?? (???? )
 
Also from (ii), the direction ratio's of the normal to the two tangent planes whose line of 
intersection is (ii) are ?? 1
,?? ,?? and ?? 2
,?? ,?? . 
Also, as these two tangent planes are perpendicular 
?                                                              ?? 1
·?? 2
+?? ·?? +?? ·?? =0 
?                                                            (?? 3
+2???? )+?? 2
+?? 2
=0                                          from (iv) 
?                                                                        2?? 2
+?? 2
+2???? =0                                                     (v) 
Now, we are to prove that the line (i) touches a parabola (to be found). So, we are to find 
the envelope of (i) which satisfies the condition (v). 
Eliminating ?? between (i) and (?? ) , the equations of the line of intersection of two tangent 
planes is : 
2?? 2
+?? 2
+2(???? +???? )?? =0,?? =0 
?                                           2(
?? ?? )
2
+2?? (
?? ?? )+(1+2?? )=0,?? =0 
It is quadratic in 
?? ?? , so its envelope is given by: 
?? 2
-4???? =0,?? =0 
?                                          (2?? )
2
-4·2(1+2?? )=0,?? =0 
?                                                                            ?? 2
=2(2?? +1),?? =0 
This is the required curve. 
6.4 Find the volume of the solid above the ???? -plane and directly below the portion 
of the elliptic paraboloid ?? ?? +
?? ?? ?? =?? which is cut off by the plane ?? =?? . 
(20i7 : 15 Marks) 
Solution: 
Equation of ?? surface, cut off cut plane 
?? 2
+
?? 2
4
 =9;?? =9
 i.e.,                                                                 
?? 2
9
+
?? 2
36
 =1;?? =9
 
 
Making the transformation, 
?? =3?? cos ?? ?? =6?? sin ?? 
?? :0 to 1;?? :0 to 2?? 
Page 5


Edurev123 
6. Paraboloid and it Properties 
6.1 Show that the plane ?? ?? +?? ?? +?? ?? +
?? ?? =?? touches the paraboloid ?? ?? ?? +?? ?? ?? =
???? ?? and find the point of contact. 
(2010 : 20 Marks) 
Solution: 
Given plane 
?? =3?? +3?? +7?? +
5
2
=0 (??)
 
and 
 Paraboloid =3?? 2
+4?? 2
=10?? 
Now, let point of contact be (?? 1
,?? 1
,?? 1
) . 
? If given plane touches paraboloid at this point of contact, then the plane should be 
tangent plane to the paraboloid. 
Now, equation of tangent plane at (?? 1
,?? 1
,?? 1
) is 
3?? ?? 1
+4?? ?? 1
=
10(?? +?? 1
)
2
 
?                                    3?? ?? 1
+4?? ?? 1
-5?? -5?? 1
=0 
Comparing (1) and (2), we get 
      
3
3?? 1
=
4
4?? 1
=
7
-5
=
5/2
-521
 
?                                                               
3
3?? 1
=
-7
5
??? 1
=
-5
7
 
4
4?? 1
=
7
-5
??? 1
=
-5
7
 
?                                                                
7
-5
=
5
-5×221
??? 1
=
5
14
 
?              Point of contact (?? 1
,?? 1
,?? 1
) =(
-5
7
,
-5
7
,
5
14
) 
6.2. Show that the locus of a point from which the three mutually perpendicular 
tangent lines can be drawn to the paraboloid ?? ?? +?? ?? +?? ?? =?? is 
?? ?? +?? ?? +?? ?? =?? 
(2012 : 20 Marks) 
Solution: 
Let ?? (?? 1
,?? 1
,?? 1
) be the point from which three mutually perpendicular lines can be drawn 
to the paraboloid 
?? 2
+?? 2
+2?? =0 (??) 
Then, the enveloping cone of (i) with the vertex at ?? 
(?? 1
,?? 1
,?? 1
) is 
?? ?? 1
 =?? 2
(???? )
?? =?? 2
+?? 2
+2?? ?? 1
 =?? 1
2
+?? 1
2
+2?? 1
?? =?? ?? 1
+?? ?? 1
+?? +?? 1
 
? from (ii), we have 
(?? 2
+?? 2
+2?? )(?? 1
2
+?? 1
2
+2?? 1
)=(?? ?? 1
+?? ?? 1
+(?? +?? 1
))
2
 
For three mutually perpendicular generators, coefficient of ?? 2
+ coefficient of ?? 2
+ 
coefficient of ?? 2
=0 
or 
?? 1
2
+?? 1
2
+4?? 1
-1 =0
?? 1
2
+?? 1
2
+4?? 1
 =0
 
? Locus of (?? 1
,?? 1
,?? 1
) is  
         ?? 2
+?? 2
+4?? =1 
6.3 Two perpendicular tangent planes to the parabolold ?? ?? +?? ?? =?? ?? intersect in a 
straight line in the plane ?? =?? . Obtain the curve to which this straight line 
touches. 
(2015 : 13 Marks) 
Solution: 
Let the line of intersection of the two planes be: 
???? +???? =?? ,?? =0 (??) 
Since this lies on the plane ?? =0 (given). 
? Equation of the plane through the line (i) is 
(???? +???? -?? )+???? =0
???? +???? +???? =?? (???? ) 
If the plane (ii) touches the paraboloid, then 
?? 2
?? 2
+
?? 2
?? 2
+
2????
?? =0 (condition)  
i.e., 
?? 2
+?? 2
+2???? =0 (?????? ) 
This being quadratic in ?? , gives two values of ?? 1
 say ?? 1
 and ?? 2
 such that 
?? 1
·?? 2
=
?? 2
+2????
?? (???? )
 
Also from (ii), the direction ratio's of the normal to the two tangent planes whose line of 
intersection is (ii) are ?? 1
,?? ,?? and ?? 2
,?? ,?? . 
Also, as these two tangent planes are perpendicular 
?                                                              ?? 1
·?? 2
+?? ·?? +?? ·?? =0 
?                                                            (?? 3
+2???? )+?? 2
+?? 2
=0                                          from (iv) 
?                                                                        2?? 2
+?? 2
+2???? =0                                                     (v) 
Now, we are to prove that the line (i) touches a parabola (to be found). So, we are to find 
the envelope of (i) which satisfies the condition (v). 
Eliminating ?? between (i) and (?? ) , the equations of the line of intersection of two tangent 
planes is : 
2?? 2
+?? 2
+2(???? +???? )?? =0,?? =0 
?                                           2(
?? ?? )
2
+2?? (
?? ?? )+(1+2?? )=0,?? =0 
It is quadratic in 
?? ?? , so its envelope is given by: 
?? 2
-4???? =0,?? =0 
?                                          (2?? )
2
-4·2(1+2?? )=0,?? =0 
?                                                                            ?? 2
=2(2?? +1),?? =0 
This is the required curve. 
6.4 Find the volume of the solid above the ???? -plane and directly below the portion 
of the elliptic paraboloid ?? ?? +
?? ?? ?? =?? which is cut off by the plane ?? =?? . 
(20i7 : 15 Marks) 
Solution: 
Equation of ?? surface, cut off cut plane 
?? 2
+
?? 2
4
 =9;?? =9
 i.e.,                                                                 
?? 2
9
+
?? 2
36
 =1;?? =9
 
 
Making the transformation, 
?? =3?? cos ?? ?? =6?? sin ?? 
?? :0 to 1;?? :0 to 2?? 
                            
?? (?? ,?? )
?? (?? ,?? )
 =|
??? ??? ??? ??? ??? ??? ??? ??? |=|
3cos ?? -3?? sin ?? 6sin ?? 6?? cos ?? |
 =18?? ?? =??
?? ????? ×???? =? ?
2?? ?? =0
?? ?
1
?? =0
?(9?? 2
)18??????????                              (?? =?? 2
+
?? 2
4
) 
 =9×18? ?
2?? 0
????? ? ?
1
0
??? 3
????
 =9×18×2?? ×
1
4
=81?? 
6.5 Reduce the following equation to the standard form and hence determine the 
nature of the coincoid: 
?? ?? +?? ?? +?? ?? -???? -???? -???? -?? ?? -?? ?? -?? ?? +???? =?? 
(2017 ; 15 Marks) 
Solution: 
Comparing with 
?? (?? ,?? ,?? ) =?? ?? 2
+?? ?? 2
+?? ?? 2
+2?????? +2?????? +2h???? +2???? +2???? +2???? +?? =0
 
The discriminating cubic is : 
                                  |
?? -?? h ?? h ?? -?? ?? ?? ?? ?? -?? |=0 or 
|
|
1-?? -
1
2
-
1
2
-
1
2
1-?? -
1
2
-
1
2
-
1
2
1-?? |
|
=0
 ?                                       4?? 3
-12?? 2
+9?? =0 or ?? (2?? -3)
2
=0
 ?                                                                      ?? =
3
2
,
3
2
,0
 
As this discriminating cube has two roots equal and third root equal to zero, so it is either 
a paraboloid of revolution or a right circular cylinder. 
The d.r.'s of the axis are given by ???? +h?? +???? =0,h?? +???? +???? =0,???? +???? +???? =0 
??.?? .,                               ?? -
?? 2
-
?? 2
=0,-
?? 2
+?? -
?? 2
=0,-
?? 2
-
?? 2
+?? =0
??.?? .,                               2?? -?? -?? =0,-?? +2?? -?? =0,-?? -?? +2?? =0
 
These gives:                           ?? =?? =?? =
1
v3
 
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