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Edurev123 
5. Application to Geometry 
5.1 A curve in space is defined by the vector equation ?? =?? 2
??ˆ+2?? ??ˆ-?? 3
??ˆ
. Determine the 
angle between the tangents to this curve at the ?? =+1 and ?? =-1. 
(2013 : 10 Marks) 
Solution: 
??  =?? 2
??ˆ+2?? ??ˆ-?? 3
??ˆ
????
????
 =2?? ??ˆ+2??ˆ-3?? 2
??ˆ
????
????
 =|
?? ?? 
????
|=v4?? 2
+4+9?? 4
??ˆ
 =
?? ?? 
????
=
?? ?? 
????
????
????
=
2+??ˆ+2??ˆ-3?? 2
??ˆ
v4?? 2
+4+9?? 4
 
where ??ˆ
 is the direction of the tangent 
??ˆ|
?? =1
=
2??ˆ+2??ˆ-3??ˆ
v17
??ˆ|
?? =-1
=
-2??ˆ+2??ˆ-3??ˆ
v17
 
Let ?? be the angle 
Then 
                                   cos ?? =??ˆ
1
·??ˆ
2
=
1
17
(2??ˆ+2??ˆ-3??ˆ
)·(-2??ˆ+2??ˆ-3??ˆ
)=
-4+4+9
17
=
9
17
 
?                                      ?? =cos
-1
 
9
17
 
5.2 Find the angle between the surfaces ?? ?? +?? ?? +?? ?? -?? =?? and ?? =?? ?? +?? ?? -?? at 
(?? ,-?? ,?? ) . 
(2015 : 10 Marks) 
Solution: 
Angle between two surfaces at a point is the angle between the normals to the surfaces 
at that point. 
Let 
Page 2


Edurev123 
5. Application to Geometry 
5.1 A curve in space is defined by the vector equation ?? =?? 2
??ˆ+2?? ??ˆ-?? 3
??ˆ
. Determine the 
angle between the tangents to this curve at the ?? =+1 and ?? =-1. 
(2013 : 10 Marks) 
Solution: 
??  =?? 2
??ˆ+2?? ??ˆ-?? 3
??ˆ
????
????
 =2?? ??ˆ+2??ˆ-3?? 2
??ˆ
????
????
 =|
?? ?? 
????
|=v4?? 2
+4+9?? 4
??ˆ
 =
?? ?? 
????
=
?? ?? 
????
????
????
=
2+??ˆ+2??ˆ-3?? 2
??ˆ
v4?? 2
+4+9?? 4
 
where ??ˆ
 is the direction of the tangent 
??ˆ|
?? =1
=
2??ˆ+2??ˆ-3??ˆ
v17
??ˆ|
?? =-1
=
-2??ˆ+2??ˆ-3??ˆ
v17
 
Let ?? be the angle 
Then 
                                   cos ?? =??ˆ
1
·??ˆ
2
=
1
17
(2??ˆ+2??ˆ-3??ˆ
)·(-2??ˆ+2??ˆ-3??ˆ
)=
-4+4+9
17
=
9
17
 
?                                      ?? =cos
-1
 
9
17
 
5.2 Find the angle between the surfaces ?? ?? +?? ?? +?? ?? -?? =?? and ?? =?? ?? +?? ?? -?? at 
(?? ,-?? ,?? ) . 
(2015 : 10 Marks) 
Solution: 
Angle between two surfaces at a point is the angle between the normals to the surfaces 
at that point. 
Let 
?? 1
=?? 2
+?? 2
+?? 2
 
and 
?? 2
=?? 2
+?? 2
-?? 
Then                                                 grad (?? 1
)=2???? +2???? +2???? 
                                                           grad (?? 2
)=2???? +2???? -?? 
??????                                      ?? 1
=grad ?? 1
 at point (2,-1,2)=4?? -2?? +4?? 
?? 2
=grad ?? 2
 at point (2,-1,2)=4?? -2?? -4?? 
The vectors ?? 1
 and ?? 2
 are along normals to the two surfaces at the point (2,-1,2) . If ?? is 
the angle between these two vectors then, 
cos ?? =
?? 1
·?? 2
|?? 1
||?? 2
|
 =
16+4-4
v16+4+16v16+4+1
=
16
6v21
?? =cos
-1
 
8
3v21
 
5.3 Find the values of ?? and ?? so that the surfaces ?? ?? ?? -?????? =(?? +?? )?? and ?? ?? ?? ?? +
?? ?? =?? may intersect orthogonally at (?? ,-?? ,?? ) . 
(2015 : 12 Marks) 
Solution: 
Let 
?? 1
 =?? ?? 2
-(?? +2)?? -?????? ?? 2
 =4?? 2
?? +?? 3
grad ?? 1
 =??? 1
=(2???? -?? -2)?? -?????? -?????? ??? 2
 =8?????? +4?? 2
?? +3?? 2
?? 
At point (1,-1,2) 
?? 1
 =(?? -2)?? -2???? +????
?? 2
 =-8?? +4?? +12?? ?? 1
·?? 2
 =0
-8(?? -2)-8?? +12?? =0
  (intersect orthogonally)  
                                            -8?? +4?? =-16?2?? -?? =4                                        (??)   
Point (1,-1,2) lies on surface ?? ?? 2
-?????? =(?? +2)?? 
Page 3


Edurev123 
5. Application to Geometry 
5.1 A curve in space is defined by the vector equation ?? =?? 2
??ˆ+2?? ??ˆ-?? 3
??ˆ
. Determine the 
angle between the tangents to this curve at the ?? =+1 and ?? =-1. 
(2013 : 10 Marks) 
Solution: 
??  =?? 2
??ˆ+2?? ??ˆ-?? 3
??ˆ
????
????
 =2?? ??ˆ+2??ˆ-3?? 2
??ˆ
????
????
 =|
?? ?? 
????
|=v4?? 2
+4+9?? 4
??ˆ
 =
?? ?? 
????
=
?? ?? 
????
????
????
=
2+??ˆ+2??ˆ-3?? 2
??ˆ
v4?? 2
+4+9?? 4
 
where ??ˆ
 is the direction of the tangent 
??ˆ|
?? =1
=
2??ˆ+2??ˆ-3??ˆ
v17
??ˆ|
?? =-1
=
-2??ˆ+2??ˆ-3??ˆ
v17
 
Let ?? be the angle 
Then 
                                   cos ?? =??ˆ
1
·??ˆ
2
=
1
17
(2??ˆ+2??ˆ-3??ˆ
)·(-2??ˆ+2??ˆ-3??ˆ
)=
-4+4+9
17
=
9
17
 
?                                      ?? =cos
-1
 
9
17
 
5.2 Find the angle between the surfaces ?? ?? +?? ?? +?? ?? -?? =?? and ?? =?? ?? +?? ?? -?? at 
(?? ,-?? ,?? ) . 
(2015 : 10 Marks) 
Solution: 
Angle between two surfaces at a point is the angle between the normals to the surfaces 
at that point. 
Let 
?? 1
=?? 2
+?? 2
+?? 2
 
and 
?? 2
=?? 2
+?? 2
-?? 
Then                                                 grad (?? 1
)=2???? +2???? +2???? 
                                                           grad (?? 2
)=2???? +2???? -?? 
??????                                      ?? 1
=grad ?? 1
 at point (2,-1,2)=4?? -2?? +4?? 
?? 2
=grad ?? 2
 at point (2,-1,2)=4?? -2?? -4?? 
The vectors ?? 1
 and ?? 2
 are along normals to the two surfaces at the point (2,-1,2) . If ?? is 
the angle between these two vectors then, 
cos ?? =
?? 1
·?? 2
|?? 1
||?? 2
|
 =
16+4-4
v16+4+16v16+4+1
=
16
6v21
?? =cos
-1
 
8
3v21
 
5.3 Find the values of ?? and ?? so that the surfaces ?? ?? ?? -?????? =(?? +?? )?? and ?? ?? ?? ?? +
?? ?? =?? may intersect orthogonally at (?? ,-?? ,?? ) . 
(2015 : 12 Marks) 
Solution: 
Let 
?? 1
 =?? ?? 2
-(?? +2)?? -?????? ?? 2
 =4?? 2
?? +?? 3
grad ?? 1
 =??? 1
=(2???? -?? -2)?? -?????? -?????? ??? 2
 =8?????? +4?? 2
?? +3?? 2
?? 
At point (1,-1,2) 
?? 1
 =(?? -2)?? -2???? +????
?? 2
 =-8?? +4?? +12?? ?? 1
·?? 2
 =0
-8(?? -2)-8?? +12?? =0
  (intersect orthogonally)  
                                            -8?? +4?? =-16?2?? -?? =4                                        (??)   
Point (1,-1,2) lies on surface ?? ?? 2
-?????? =(?? +2)?? 
?                                                            ?? +2?? =(?? +2)??? =1
? 2?? -1=4??? =
5
2
 
5.4 Find the curvature vector and its magnitude at any point ??? =(?? ) of the curve 
??? =(?? ?????? ?? ,?? ?????? ?? , a). Show that the locus of the feet of the perpendicular from the 
origin to the tangent is a curve that completely lies on the hyperboloid ?? ?? +?? ?? -
?? ?? =?? ?? . 
(2017 : 16 Marks) 
Solution: 
?? =?? cos ???? +?? sin ???? +?????? 
?? ?? 
????
=-?? sin ???? +?? cos ???? +???? 
????
????
=|
????
????
|=v2?? 
?? =
?? ?? 
????
=
?? ?? 
????
????
 
 =
1
v2
(-sin ???? +cos ???? +?? )
?? ?? 
????
 =
1
v2
(-cos ?? ??ˆ-sin ?? ??ˆ)
?????????????????? ???????????? ,                       ??? 
 =
?? ?? 
????
=
?? ?? /????
???? /????
 =
1
v2?? ·
1
v2
(-cos ???? -sin ???? )
 =
-cos ????
2?? ·
sin ????
2?? 
Magnitude, 
|?? |=
1
2?? 
Locus of feet of perpendicular from origin to tangent 
Page 4


Edurev123 
5. Application to Geometry 
5.1 A curve in space is defined by the vector equation ?? =?? 2
??ˆ+2?? ??ˆ-?? 3
??ˆ
. Determine the 
angle between the tangents to this curve at the ?? =+1 and ?? =-1. 
(2013 : 10 Marks) 
Solution: 
??  =?? 2
??ˆ+2?? ??ˆ-?? 3
??ˆ
????
????
 =2?? ??ˆ+2??ˆ-3?? 2
??ˆ
????
????
 =|
?? ?? 
????
|=v4?? 2
+4+9?? 4
??ˆ
 =
?? ?? 
????
=
?? ?? 
????
????
????
=
2+??ˆ+2??ˆ-3?? 2
??ˆ
v4?? 2
+4+9?? 4
 
where ??ˆ
 is the direction of the tangent 
??ˆ|
?? =1
=
2??ˆ+2??ˆ-3??ˆ
v17
??ˆ|
?? =-1
=
-2??ˆ+2??ˆ-3??ˆ
v17
 
Let ?? be the angle 
Then 
                                   cos ?? =??ˆ
1
·??ˆ
2
=
1
17
(2??ˆ+2??ˆ-3??ˆ
)·(-2??ˆ+2??ˆ-3??ˆ
)=
-4+4+9
17
=
9
17
 
?                                      ?? =cos
-1
 
9
17
 
5.2 Find the angle between the surfaces ?? ?? +?? ?? +?? ?? -?? =?? and ?? =?? ?? +?? ?? -?? at 
(?? ,-?? ,?? ) . 
(2015 : 10 Marks) 
Solution: 
Angle between two surfaces at a point is the angle between the normals to the surfaces 
at that point. 
Let 
?? 1
=?? 2
+?? 2
+?? 2
 
and 
?? 2
=?? 2
+?? 2
-?? 
Then                                                 grad (?? 1
)=2???? +2???? +2???? 
                                                           grad (?? 2
)=2???? +2???? -?? 
??????                                      ?? 1
=grad ?? 1
 at point (2,-1,2)=4?? -2?? +4?? 
?? 2
=grad ?? 2
 at point (2,-1,2)=4?? -2?? -4?? 
The vectors ?? 1
 and ?? 2
 are along normals to the two surfaces at the point (2,-1,2) . If ?? is 
the angle between these two vectors then, 
cos ?? =
?? 1
·?? 2
|?? 1
||?? 2
|
 =
16+4-4
v16+4+16v16+4+1
=
16
6v21
?? =cos
-1
 
8
3v21
 
5.3 Find the values of ?? and ?? so that the surfaces ?? ?? ?? -?????? =(?? +?? )?? and ?? ?? ?? ?? +
?? ?? =?? may intersect orthogonally at (?? ,-?? ,?? ) . 
(2015 : 12 Marks) 
Solution: 
Let 
?? 1
 =?? ?? 2
-(?? +2)?? -?????? ?? 2
 =4?? 2
?? +?? 3
grad ?? 1
 =??? 1
=(2???? -?? -2)?? -?????? -?????? ??? 2
 =8?????? +4?? 2
?? +3?? 2
?? 
At point (1,-1,2) 
?? 1
 =(?? -2)?? -2???? +????
?? 2
 =-8?? +4?? +12?? ?? 1
·?? 2
 =0
-8(?? -2)-8?? +12?? =0
  (intersect orthogonally)  
                                            -8?? +4?? =-16?2?? -?? =4                                        (??)   
Point (1,-1,2) lies on surface ?? ?? 2
-?????? =(?? +2)?? 
?                                                            ?? +2?? =(?? +2)??? =1
? 2?? -1=4??? =
5
2
 
5.4 Find the curvature vector and its magnitude at any point ??? =(?? ) of the curve 
??? =(?? ?????? ?? ,?? ?????? ?? , a). Show that the locus of the feet of the perpendicular from the 
origin to the tangent is a curve that completely lies on the hyperboloid ?? ?? +?? ?? -
?? ?? =?? ?? . 
(2017 : 16 Marks) 
Solution: 
?? =?? cos ???? +?? sin ???? +?????? 
?? ?? 
????
=-?? sin ???? +?? cos ???? +???? 
????
????
=|
????
????
|=v2?? 
?? =
?? ?? 
????
=
?? ?? 
????
????
 
 =
1
v2
(-sin ???? +cos ???? +?? )
?? ?? 
????
 =
1
v2
(-cos ?? ??ˆ-sin ?? ??ˆ)
?????????????????? ???????????? ,                       ??? 
 =
?? ?? 
????
=
?? ?? /????
???? /????
 =
1
v2?? ·
1
v2
(-cos ???? -sin ???? )
 =
-cos ????
2?? ·
sin ????
2?? 
Magnitude, 
|?? |=
1
2?? 
Locus of feet of perpendicular from origin to tangent 
????
????? 
 =?? -(?? ·?? )?? 
?? = Foot of perpendicular 
?? ·??  =(?? cos ???? +?? sin ???? +?????? )·(-sin ???? +cos ?? +?? )
1
v2
 =
????
v2
?? -(?? ·?? )??  =(?? cos ???? +?? sin ???? +?????? )-
????
v2
·
1
v2
(-sin ???? +cos ?? +?? )
 =(?? cos ?? +
???? sin ?? 2
)?? +(?? sin ?? -
???? cos ?? 2
)?? +
?? 2
?? 
 
Let foot of perpendicular be ?? (?? ,?? ,?? ) 
 
?? =?? cos ?? +
???? sin ?? 2
?? =?? sin ?? -
???? cos ?? 2
?? =
????
2
?                                          ?? 2
+?? 2
-?? 2
 =?? 2
+
?? 2
?? 2
4
-
?? 2
?? 2
4
=?? 2
 
?? 2
+?? 2
-?? 2
=?? 2
 is the required locus. 
5.5 Find the angle between tangent at general point of curve whose equations are 
?? =?? ?? ,?? =?? ?? ?? , ?? =?? ?? ?? and the line ?? -?? =?? =?? . 
(2018: 10 marks) 
Solution: 
The radius vector of given curve, ??  can be written as 
Page 5


Edurev123 
5. Application to Geometry 
5.1 A curve in space is defined by the vector equation ?? =?? 2
??ˆ+2?? ??ˆ-?? 3
??ˆ
. Determine the 
angle between the tangents to this curve at the ?? =+1 and ?? =-1. 
(2013 : 10 Marks) 
Solution: 
??  =?? 2
??ˆ+2?? ??ˆ-?? 3
??ˆ
????
????
 =2?? ??ˆ+2??ˆ-3?? 2
??ˆ
????
????
 =|
?? ?? 
????
|=v4?? 2
+4+9?? 4
??ˆ
 =
?? ?? 
????
=
?? ?? 
????
????
????
=
2+??ˆ+2??ˆ-3?? 2
??ˆ
v4?? 2
+4+9?? 4
 
where ??ˆ
 is the direction of the tangent 
??ˆ|
?? =1
=
2??ˆ+2??ˆ-3??ˆ
v17
??ˆ|
?? =-1
=
-2??ˆ+2??ˆ-3??ˆ
v17
 
Let ?? be the angle 
Then 
                                   cos ?? =??ˆ
1
·??ˆ
2
=
1
17
(2??ˆ+2??ˆ-3??ˆ
)·(-2??ˆ+2??ˆ-3??ˆ
)=
-4+4+9
17
=
9
17
 
?                                      ?? =cos
-1
 
9
17
 
5.2 Find the angle between the surfaces ?? ?? +?? ?? +?? ?? -?? =?? and ?? =?? ?? +?? ?? -?? at 
(?? ,-?? ,?? ) . 
(2015 : 10 Marks) 
Solution: 
Angle between two surfaces at a point is the angle between the normals to the surfaces 
at that point. 
Let 
?? 1
=?? 2
+?? 2
+?? 2
 
and 
?? 2
=?? 2
+?? 2
-?? 
Then                                                 grad (?? 1
)=2???? +2???? +2???? 
                                                           grad (?? 2
)=2???? +2???? -?? 
??????                                      ?? 1
=grad ?? 1
 at point (2,-1,2)=4?? -2?? +4?? 
?? 2
=grad ?? 2
 at point (2,-1,2)=4?? -2?? -4?? 
The vectors ?? 1
 and ?? 2
 are along normals to the two surfaces at the point (2,-1,2) . If ?? is 
the angle between these two vectors then, 
cos ?? =
?? 1
·?? 2
|?? 1
||?? 2
|
 =
16+4-4
v16+4+16v16+4+1
=
16
6v21
?? =cos
-1
 
8
3v21
 
5.3 Find the values of ?? and ?? so that the surfaces ?? ?? ?? -?????? =(?? +?? )?? and ?? ?? ?? ?? +
?? ?? =?? may intersect orthogonally at (?? ,-?? ,?? ) . 
(2015 : 12 Marks) 
Solution: 
Let 
?? 1
 =?? ?? 2
-(?? +2)?? -?????? ?? 2
 =4?? 2
?? +?? 3
grad ?? 1
 =??? 1
=(2???? -?? -2)?? -?????? -?????? ??? 2
 =8?????? +4?? 2
?? +3?? 2
?? 
At point (1,-1,2) 
?? 1
 =(?? -2)?? -2???? +????
?? 2
 =-8?? +4?? +12?? ?? 1
·?? 2
 =0
-8(?? -2)-8?? +12?? =0
  (intersect orthogonally)  
                                            -8?? +4?? =-16?2?? -?? =4                                        (??)   
Point (1,-1,2) lies on surface ?? ?? 2
-?????? =(?? +2)?? 
?                                                            ?? +2?? =(?? +2)??? =1
? 2?? -1=4??? =
5
2
 
5.4 Find the curvature vector and its magnitude at any point ??? =(?? ) of the curve 
??? =(?? ?????? ?? ,?? ?????? ?? , a). Show that the locus of the feet of the perpendicular from the 
origin to the tangent is a curve that completely lies on the hyperboloid ?? ?? +?? ?? -
?? ?? =?? ?? . 
(2017 : 16 Marks) 
Solution: 
?? =?? cos ???? +?? sin ???? +?????? 
?? ?? 
????
=-?? sin ???? +?? cos ???? +???? 
????
????
=|
????
????
|=v2?? 
?? =
?? ?? 
????
=
?? ?? 
????
????
 
 =
1
v2
(-sin ???? +cos ???? +?? )
?? ?? 
????
 =
1
v2
(-cos ?? ??ˆ-sin ?? ??ˆ)
?????????????????? ???????????? ,                       ??? 
 =
?? ?? 
????
=
?? ?? /????
???? /????
 =
1
v2?? ·
1
v2
(-cos ???? -sin ???? )
 =
-cos ????
2?? ·
sin ????
2?? 
Magnitude, 
|?? |=
1
2?? 
Locus of feet of perpendicular from origin to tangent 
????
????? 
 =?? -(?? ·?? )?? 
?? = Foot of perpendicular 
?? ·??  =(?? cos ???? +?? sin ???? +?????? )·(-sin ???? +cos ?? +?? )
1
v2
 =
????
v2
?? -(?? ·?? )??  =(?? cos ???? +?? sin ???? +?????? )-
????
v2
·
1
v2
(-sin ???? +cos ?? +?? )
 =(?? cos ?? +
???? sin ?? 2
)?? +(?? sin ?? -
???? cos ?? 2
)?? +
?? 2
?? 
 
Let foot of perpendicular be ?? (?? ,?? ,?? ) 
 
?? =?? cos ?? +
???? sin ?? 2
?? =?? sin ?? -
???? cos ?? 2
?? =
????
2
?                                          ?? 2
+?? 2
-?? 2
 =?? 2
+
?? 2
?? 2
4
-
?? 2
?? 2
4
=?? 2
 
?? 2
+?? 2
-?? 2
=?? 2
 is the required locus. 
5.5 Find the angle between tangent at general point of curve whose equations are 
?? =?? ?? ,?? =?? ?? ?? , ?? =?? ?? ?? and the line ?? -?? =?? =?? . 
(2018: 10 marks) 
Solution: 
The radius vector of given curve, ??  can be written as 
??  =3??ˆ+3?? 2
??ˆ+3?? 3
??ˆ
?? ?? 
????
 =3??ˆ+6??ˆ+9?? 2
??ˆ
????
????
 =|
?? ?? 
????
|=v9+36?? 2
+81?? 4
=3v1+4?? 2
+9?? 4
 
Tangent vector, 
??? 
=
?? ?? 
????
=
?? ?? /????
???? /????
=
?? 
3v1+4?? 2
+9?? 4
×(3??ˆ+6?? ??ˆ) 
?                                    ??? 
=
??ˆ+2?? ??ˆ+3?? 2
??ˆ
v1+4?? 2
+9?? 4
 
The given line can be written as 
?? 1
=
?? 0
=
?? 1
 
? angle between line and tangent vector, ?? 
cos ?? =
1×1+2?? ×0+3?? 2
×1
v2·v1+4?? 2
+9?? 4
=
1+3?? 2
v2·v1+4?? 2
+9?? 4
?? =cos
-1
 (
1+3?? 2
v2·v1+4?? 2
+9?? 4
)
 
5.6 Find the circulation of ???? 
 round the curve ?? , where ???? 
=(?? ?? +?? ?? )??ˆ+(?? ?? -?? ?? )??ˆ 
and ?? is the curve ?? =?? ?? from (?? ,?? ) to (?? ,?? ) and the curve ?? ?? =?? from (?? ,?? ) to 
(?? ,?? ) . 
(2019: 15 Marks) 
Solution: 
Here the closed curve ' ?? consists of arcs ?????? and ?????? . Let ?? 1
 denote the arc ?????? and 
?? 2
 denote arc ?????? . Along ?? 1
, we have ?? =?? 2
 so that ???? =2?????? and ?? varies from 0?1 
along ?? 2
, we have ?? =?? 2
 so that ???? = 2?????? and ?? varies from 1 to also. 
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